T H E C O L L E G E O F A E R O N A U T I C S
O 7 AllR IQRP C R A N F I E L D
THE INFLUENCE O F NITROGEN O'N THE PROPERTIES
OF MILD STEEL WELD METAL DEPOSITED FROM
COVERED ELECTRODES
CoA REPORT M A T No. 4
THE COLLEGE OF AERONAUTICS CRANFIELD
The Influence of Nitrogen on the Properties of Mild Steel Weid Metal Deposited from Covered Electrodes
by
w e r e found to deposit l o w e r n i t r o g e n c o n t e n t s than acid o r r u t i l e d e p o s i t s . D r y i n g at t e m p e r a t u r e s up to 800°C was found to i n c r e a s e t h e n i t r o g e n content and d e c r e a s e t h e h y d r o g e n content of acid and r u t i l e weld d e p o s i t s , but s u p p l e -m e n t a r y gas shielding with CO2, A o r H2 r e d u c e d the n i t r o g e n content t o v e r y low l e v e l s . T h e n i t r o g e n content was a l s o influenced by the t h i c k n e s s of the e l e c t r o d e coating and t h e c o m p o s i t i o n of the weld d e p o s i t .
I n c r e a s e in n i t r o g e n content was found to i n c r e a s e the yield and t e n s i l e s t r e n g t h s of mild s t e e l weld d e p o s i t s for both r o o m and elevated t e m p e r a t u r e s at t h e e x p e n s e of d u c t i l i t y and notch t o u g h n e s s .
N i t r o g e n was a l s o found to influence t h e incidence of f i s s u r i n g and c r a c k i n g in weld d e p o s i t s : with n i t r o g e n c o n t e n t s of 0.007% no f i s s u r i n g was found even in the p r e s e n c e of high h y d r o g e n c o n t e n t s .
CONTENTS
Page
Summary
Introduction 1
A Comparison of Oxygen, Hydrogen and Nitrogen Solubilities 3
Experimental 8 3.1 Nitrogen content of weld metal. 8
3.2 The effect of nitrogen on the mechanical properties 9 of weld metals
3.3 Effect of nitrogen on the susceptibility of weld metal 9 to microfissuring
3.4 Effect of nitrogen on porosity in weld metal 10
Discussion 11
Conclusions 13
References 14
Tables I - XIV
1. Introduction
The coverings used for shielded metal arc electrodes have several functions, both technical and practical, but the major function of all coverings is to protect the weld against atmospheric contamination. In general, adequate protection is given by all modern electrode coatings, but, nevertheless, the degree of protection varies considerably with the type of coating.
During the a r c welding of mild steel, three gases can react with the molten weld metal: oxygen, nitrogen and hydrogen. Additionally oxidation of the weld metal may occur by reaction with gaseous or liquid oxides. The result of these reactions will be an increase in oxygen, nitrogen and hydrogen content of the weld matal.
The influence of the type of electrode covering on the gas content of mild steel weld metal is generally understood, although the majority of published work has concentrated on hydrogen. Weld metal deposited from basic electrodes usually contains the smallest quantities of hydrogen, oxygen and nitrogen. Acid electrodes give weld deposits with large amounts of hydrogen and oxygen, the latter element aiainly being present as oxides. Rutile electrodes are known to deposit weld metal with higher hydrogen, nitrogen and oxygen contents than basic electrodes.
In general, rutile weld deposits also contain greater amounts of nitrogen and smaller amounts of oxygen than weld metal deposited by acid electrodes. Cellulosic electrodes, which rely largely on a hydrogen gaseous shield to protect the weld from atmospheric contamination, give weld metal with the greatest hydrogen content and with comparatively high oxygen, but low nitrogen contents, in comparison with weld deposits from acid and rutile electrodes. On occasions, weld metal deposited from cellulosic electrodes may have a lower nitrogen content than deposits from basic electrodes.
The type of electrode covering can have a considerable effect on weld metal properties. Basic coatings give the best overall properties and these properties are generally attributed to the low hydrogen content of basic weld metal. However, cellulosic deposits with high hydrogen content can have better ductility and, in particular, better low temperature impact properties than rutile deposits with a lower hydrogen content. It is suggested that these superior properties of cellulosic weld deposits are a direct result of their low nitrogen contents.
The nitrogen content of cast or hot wrought low carbon steels is generally no greater than 0.005%, which is considerably lower than the nitrogen content of most weld metal. Thus basic weld metal contains no more than 0.01% nitrogen but rutile deposits can contain more than 0.02%. In contrast, submerged arc weld deposits contain only 0.003% nitrogen. Nitrogen is known to strongly influence the properties of steel and must be expected to have a marked effect on the properties of weld metal deposited by covered electrodes.
Most welding processes have been developed with the purpose of elimin-ating harmful gases from the weld pool. The most detrimental gases are generally considered to be hydrogen, oxygen and nitrogen, although most attention has, in the past been concentrated on the influence of hydrogen.
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-It i s , therefore, necessary to consider the form in which these gases can exist in the weld metal.
Hydrogen does not form stable compounds in steel and in most cases it is retained in weld metal in harmless quantities. In fact, welding processes exist in which hydrogen forms the whole, or major part of the shielding medium, for example, the atomic hydrogen p r o c e s s . Hydrogen can be d e t r i -mental in the heat affected zones of low alloy s t e e l s , when it can lead to cold cracking. It is l e s s harmful in mild steel welds, however, particularly with s m a l l e r thicknesses and under conditions of low restraint. There is evidence that hydrogen can cause fissuring in mild steel weld metal, but the significance of small fissures with regard to weld properties is still uncertain. In cases where hydrogen is likely to be harmful it is comparatively easy to control its content in welds by the use of low hydrogen electrodes or welding processes.
Oxygen will lead to the formation of oxides in the molten weld pool. These will be largely transferred to the slag but some will remain trapped in non-metallic (often silicate) inclusions. The oxygen content of steel weld deposits is readily controlled by the introduction of a sufficient quantity of deoxidants into the weld pool. Control is readily oxercised with the submerged a rc or CO2 welding processes or when welding with basic electrodes but is more difficult with rutile or acid electrodes.
The majority of processes for welding steel offer better protection against nitrogen than oxygen, although this development has been accidental rather than intentional. In particular, welding in atmospheres of carbon dioxide or steam do not prevent oxidation but they do largely prevent reaction with nitrogen. Acid and rutile covered electrodes also offer little opportunity for thorough deoxidation of the weld metal.
However, when nitrogen does occur in the weld pool, its low diffusion r a t e (compared for example with that of hydrogen) offers little opportunity for evolution of the nitrogen during solidification. The nitrogen dissolved in the molten weld pool does not react with the liquid metal but, after solidification, it will react to form non-gaseous nitrides. In the case of low carbon steels which do not contain effective nitride-formers the nitrogen will largely remain in solid solution in the weld metal. Additionally, since nitrides form only after solidification these nitrides remain in the weld metal and a r e not t r a n s -ferred to the slag, so that the total nitrogen content of the weld is unchanged. As a result, the ductility and low temperature impact strength of the weld metal remain unchanged.
Despite this problem of nitrogen contamination several patents have
recently been granted for filler wires which can be used in air and a r e alleged to give satisfactory mechanical properties. Thus the ar c welding processes a r e reverting to the point at which they started i . e . welding with bare wires. However, these new filler wires for use in air differ considerably from the fence wire that was used some sixty years ago, in that they contain consider-able amounts of aluminium, titanium and zirconium. These elements are effective deoxidants and, it must be presumed, can react effectively with nitrogen to reduce its harmful effects.
In the present work which was carried out partly at the Polish Welding Research Institute, Gliwice, and partly at the College of Aeronautics, Cranfield, the influence of nitrogen on the soundness and properties of mild steel weld metal deposited from manual covered electrodes has been studied. Particular attention has been paid to impact properties, tensile properties, and the
incidence of fissuring and porosity.
Although the work is by no means complete the results do show that nitrogen has a considerable effect upon the soundness and properties of mild steel weld deposits and indicate a need for a r e - a s s e s s m e n t of the role of nitrogen in a rc welding.
2. A Comparison of Oxygen, Hydrogen and Nitrogen Solubilities
Nitrogen, hydrogen and oxygen dissolve in the weld metal in the same way that they do in the molten steel, i . e . in the atomic form. However, there a r e differences in solubilities of each particular gas, with the solution of nitrogen and hydrogen showing certain similarity in the first stage of the p r o c e s s . The solubility of oxygen as a solution in the liquid iron is very low and amounts to no m o r e than 0.005 - 0.003%, but the solubility of oxygen as FeO in the liquid iron is relatively high and amounts to about 0.2% by weight of oxygen. In the liquid iron therefore, oxygen can occur in two forms: as a solution of oxygen in iron, [O], and as a solution of iron oxide in iron,
[FeO]. The solubility of oxygen in the liquid steel depends on the content of the deoxidising elements in the steel. They usually decrease the concentration of [FeO] without affecting oxygen present as [O] in the liquid steel. The highest content of [FeO] occurs in mild, rimming s t e e l s . These steels, when overheated in the liquid condition, show an increase in the [FeO] concentration. As the temperature of liquid rimntiing steel is lowered the oxygen solubility in the steel decreases and the following deoxidation reaction occurs:
FeO + C ^ F e + CO (1)
together with a precipitation of iron oxide from the steel. The solubility of oxygen decreases with increase of carbon content in steel.
The highest content of oxygen occurs in welds made with bare electrodes in air; it can amount up to about 0,2%. In welds made with acid and rutile electrodes the oxygen content is also relatively high, up to about 0.1%, because these welds contain only manganese as a deoxidant. The oxygen content in welds made with basic electrodes is considerably lower, about 0,05%, because such welds contain more effective deoxidants. Thus in addition to manganese, elements such as silicon and titanium or aluminium are present in most c a s e s .
The solubility of nitrogen in iron and steel and therefore in welds differs considerably from that of oxygen. While the solution of oxygen in liquid iron may be expressed by the equation:
which d e s c r i b e s t h e f o r m a t i o n and solution of i r o n oxide in t h e liquid s t e e l , t h e e x p r e s s i o n of n i t r o g e n s o l u t i o n in a s i m i l a r way i s not p o s s i b l e , b e c a u s e n i t r o g e n d o e s not f o r m c h e m i c a l c o m p o u n d s with liquid s t e e l s .
T h e r e a c t i o n of n i t r o g e n solution in s t e e l s :
N„ + F e ^ = 2 N in Fe_ (3)
w h e r e F e i s liquid i r o n , and N in F e ^ i s n i t r o g e n solution in liquid i r o n ; L L
i s a p h y s i c a l p r o c e s s of a t o m i c n i t r o g e n solution in i r o n . In a s i m i l a r way h y d r o g e n d i s s o l v e s in liquid i r o n :
H + F e = 2 H in F e . (4)
b e c a u s e it i s a l s o a d i a t o m i c g a s which d o e s not f o r m c h e m i c a l compounds with i r o n and d i s s o l v e s in it in t h e a t o m i c f o r m .
When both t h e s e g a s e s o c c u r in t h e a t o m i c and d i a t o m i c f o r m s s i m u l t -a n e o u s l y , t h e c o n c e n t r -a t i o n of h y d r o g e n -and n i t r o g e n in i r o n will depend on p a r t i a l p r e s s u r e s of g a s e s in t h e s e p a r a t e s y s t e m s F e - N„ - N and F e - Hg - H (1): [ N ] = K^ 1 + X Ng.N (5) IH] = K^ 1 + H ^ . H (6) w h e r e : K- , K a r e c o n s t a n t s and P^, a r e t h e p a r t i a l p r e s s u r e s of a t o m i c and d i a t o m i c 2 ' N „ , N H „ . H n i t r o g e n and h y d r o g e n r e s p e c t i v e l y , i . e . P«j ^ = P + P „ ^^2' 2 " 2 * " = ^Hg ^ ^ H X i s t h e d e g r e e of d i s s o c i a t i o n of n i t r o g e n o r h y d r o g e n . When h y d r o g e n i s c o m p l e t e l y d i s s o c i a t e d i . e . x = 1 then t h e c o n c e n t r a t i o n in liquid i r o n m a y be e x p r e s s e d a s follows:
[ H ] = K 2
/ « P H ^ ^V ' ' ' =
V H (7)When X = O i . e . there is no dissociation:
[H] = K2
| < P H 2 "V " " - ^ 2 \ <«>
The same expressions occur in the case of nitrogen.
In arc welding, the partial pressure of atomic hydrogen is much higher than that of atomic nitrogen since hydrogen is nearly completely dissociated at a temperature of about 5000°C whereas nitrogen has only begun to dissociate. As the solubility of atomic gases increases directly with the partial pressure in comparison with diatomic gases, in which the solubility depends on the square root of partial p r e s s u r e , the saturation in welds with hydrogen at the same temperature can be much higher than with nitrogen.
The much higher diffusion rate of hydrogen compared to that of nitrogen also has a considerable effect on the increase of solution rate of hydrogen. This is probably related to the fact that the diameter of the hydrogen atom is much smaller than that of nitrogen (the radii of hydrogen and nitrogen atoms a r e about 0.5 A and 0.75 A respectively). On the other hand, even in the case of a considerable concentration of hydrogen compounds in the a r c , the hydrogen solubility in the weld metal pool can be controlled by combining it with fluorine to form hydrogen fluoride, HF, which is insoluble in metals.
This approach is used in submerged a r c welding. There is no such possibility when welding in an atmosphere containing nitrogen.
Under conditions of steel making, particularly in the open hearth process, hydrogen and nitrogen dissolve much less than they do in the arc welding p r o c e s s e s . The solubility of nitrogen in liquid iron is about 0.045% (1600°C and 1 atm nitrogen p r e s s u r e ) . The nitrogen content in open-hearth steels is about 0.003 - 0.006% and is therefore a few times smaller than the highest content possible in iron at 1600°C and 1 atm nitrogen p r e s s u r e . The nitrogen content in arc welds can be much higher and can amount to about 0.2% as in the case of arc welding with bare electrodes in a i r . It appears that during arc welding some additional factors operate to cause an increase of nitrogen content in welds. The hydrogen content in welds in the case of welding with rutile and acid electrodes is also much higher than that in liquid open-hearth steel, but can be explained by a very high degree of hydrogen ionization in the arc and by the presence of hydrogen cations produced in the reaction of an energy type (1):
Hg-^ 2H-^ H + H"*" + e ' (9)
Hydrogen in the form of a cation H is of a very small diameter and possesses a high kinetic energy. Such hydrogen dissolves very intensively on the cathode; a fact which has been borne out by analyses of cathode and anode metals for hydrogen content (2).
The problem of nitrogen solubility in the weld has not been explored as thoroughly as that of hydrogen. For a considerable length of time it was
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-assumed (3) that nitrogen dissolved in the weld as a result of a chemical reaction i . e . a reaction of diatomic nitrogen with the molten pool. The degree of nitrogen dissociation was not considered high enough to determine the nitrogen concentration in the pool. Still l e s s credible seemed the possib-ility of nitrogen solution in the form of cations similarly to the way hydrogen dissolves:
N_—» 2 N ^ N + N"*" + e ' (10)
since the reaction H„—> 2H absorbs only about 104 kcal/mol, whilst the reaction H —>• H + H"*" + e already requires about 418 kcal/mol, A c o r r e s -ponding reaction N„—* 2N requires about 200 kcal/mol. whilst the heat of reaction N„—> N + N + e has not been determined accurately so far. It was believed that it must be much higher than that for a reaction with hydrogen and therefore required temperatures difficult to obtain in the a r c .
Also the part played by oxygen in the solution of nitrogen was not fully appreciated whereas oxygen has now been shown to be a predominant factor in accelerating the nitrogen solution (4,5). It can now be stated that, in the solution of nitrogen in welds, the same reactions take part as in the case of hydrogen as well as reactions in which oxygen participates. Nitrogen, t h e r e -fore, dissolves by the reaction of molecules with the liquid metal from the melting electrode and weld metal pool, by the reaction of atoms with the liquid metal and also, although to a smaller extent, by solution of cations, An increase of nitrogen content in welds made in a mixture of nitrogen and oxygen, i . e . a i r , is related to the formation of a compound, NO, in the a r c which occurs more easily than the formation of atomic nitrogen from N dissociation. The part oxygen plays in the transfer of nitrogen into the weld consists in the formation of NO compound in the ar c which contains one atom of nitrogen and is therefore readily soluble in the liquid steel.
NO + Fe = [N in Fe^ ] + [FeO in Fe^ ] (11)
L L
Atomic nitrogen and hydrogen have been found to dissolve in the form of cations during welding in an investigation by Lakomski and Grigorenko (2) who divide the effect into a "chemical" and an "electrical" solution. The gases dissolve chemically on the anode and their amount corresponds to the solubility at a given temperature and partial p r e s s u r e . In addition, an "electrical" solution of gases takes place at the cathode i . e . due to the flow of positive cations to the negative cathode, the extent of solution being depend-ent on the cathode potdepend-ential drop, welding currdepend-ent, temperature of metal and partial p r e s s u r e of gas.
The total solubility of nitrogen in welds in relation to the composition and p r e s s u r e of gases as well as welding current and voltage was examined by Kobayashi et al. (4). It was found that, during welding in an atmosphere of pure nitrogen, the solubility was very large at low p r e s s u r e s and was further increased by the presence of oxygen and other oxidizing gases.
Nitrogen, oxygen and hydrogen influence their mutual solubility. The relation between mutual solubilities of hydrogen and oxygen is explored most readily. F o r the range of 1550° - 1770°C it is expressed for liquid steel by the equation: H O ^ 2H + O (12)
K = l§i-LSi (13)
« 2 ^ 13 420 whÜBt log K = — + 1.49 (14)where: K - equilibrium constant
O - oxygen concentration in steel H - hydrogen concentration in steel P - partial p r e s s u r e of steam
"2°
T - temperature
This equation indicates that the solubility of hydrogen in liquid iron is inversely proportional to the square root of the oxygen content:
2 ^ " 2 °
[H]^ = K ^— (15) [O]
It is evident also that the mutual decrease of solubilities of hydrogen and oxygen is due, among other p r o c e s s e s to the reactions of deoxidation of steel by hydrogen.
FeO + H = Fe + OH (16)
FeO 4 H^ = F e + H^O (17)
as a result of which both the content of hydrogen and oxygen will be diminished.
The determination of a relationship in mutual solubilities of nitrogen and oxygen, and nitrogen and hydrogen is a more complicated problem. The experimental evidence that the nitrogen content in welds increases in the
presence of oxygen in the a r c and decreases in the presence of hydrogen in the weld pool becomes easier to grasp if the nature of oxygen, nitrogen and hydrogen solution in steel is taken into consideration. Oxygen, when dissol-ving in the liquid steel, usually forms a solution of iron oxide, [FeO], in steel, whilst nitrogen and hydrogen form solutions of nitrogen, [N], and hydrogen, [H], in steel i . e . s i m i l a r solutions of single atoms in the liquid steel. Therefore, the solution of iron oxide in steel should not diminish the
8
-capacity for nitrogen atoms to dissolve, whereas atoms of nitrogen and hydro-gen interfere with each other because they form similar solutions; the atoms of nitrogen or hydrogen replace each other by taking the same locations in the solution in liquid steel. Oxygen and nitrogen do not react together in molten steel and for this reason do not influence their mutual solubilities, oxygen and hydrogen, however, react with each other in the process of deoxidation.
The possibility of nitrogen reacting with hydrogen with the formation of ammonia radicals should also be considered, which favours a decrease of nitrogen and hydrogen content in the weld.
Probably due to the complexity of simultaneous solutions of oxygen with nitrogen and hydrogen with nitrogen the equations in mathematical form and activity factors for simultaneous dissolving of these gases have not yet been established.
3. Experimental
Experiments have been carried out using the following electrodes.
E P 52-28 Polish acid electrodes EP 47-28 Polish rutile electrodes E P 49-29 Polish basic electrodes E 307 British rutile electrodes
The initial work was done with the Polish electrodes and, due to the limited time available for work at Cranfield, t e s t s were repeated only for a British rutile electrode. The rutile covered electrode was chosen because this type of covering was known to give the greatest amount of nitrogen in the weld metal. The main aim of the work was to determine the amounts of nitrogen occurring in weld metal deposited from different types of electrode and the influence of nitrogen on weld soundness (cracks, fissures and porosity) and properties. Some results have also been obtained on the influence of coating thickness and drying temperatures on the nitrogen content of mild steel
weld metal.
A variety of nitrogen contents in the weld metal has been achieved partly by the choice of coating but also by the use of different shielding g a s e s , i . e . COg, A, H^.
Samples of weld metal for analysis and testing were prepared from manually deposited weld metal pads o r , when testing for weld soundness, by bead on plate deposits on specially grooved mild steel blocks, according to the recommendations of the International Institute of Welding (7).
3.1 Nitrogen content of weld metal
electrodes are summarised in Table I, which also includes figures for hydro-gen and oxyhydro-gen contents where these a r e available. The reason for the spread in results for the acid (EP 52-28) and rutile (EP 47-28) electrodes is due to variation in coating thickness and composition. More typical values would be 0.021% for acid (EP52-28) 0.024% for rutile (EP 47-28, E307) and 0,012 for basic (EP 49-29) weld deposits. The highest nitrogen content occurred in welds made with rutile electrodes and the lowest for basic weld metal.
The influence of coating thickness on the nitrogen content of acid weld deposits is given in Table II, the results indicating that the thicker the coating the greater is the protection against nitrogen pick-up.
The effect of drying of electrodes prior to use has been determined for acid electrodes (Table III) and rutile electrodes (Table IV). In both cases, increasing the drying temperature increases the nitrogen content of the weld metal but decreases the hydrogen content. For rutile electrodes a slight increase in oxygen content of the weld metal also occurs with increased drying temperature. The drying of basic electrodes at temperatures up to 450°C had no effect on nitrogen content of the weld deposit.
Table V shows that weld metal from rutile electrodes which contain elements favouring the absorption of nitrogen, for example chromium and vanordium, have higher nitrogen contents than rutile mild steel deposits.
3.2 The effect of nitrogen on the mechanical properties of weld metals
The effect of nitrogen in the properties of weld metal have been invest-igated by depositing welds with various electrodes in air and in a CO shield. The effect on room temperature tensile strength is shown in Tables VI and IX, the weld metal with the lower nitrogen content having markedly lower tensile and yield strengths, whatever the electrode covering. Elevated temperature tensile properties are also affected by the nitrogen content, Table VII and Figures 1 and 2. With Increase in tensile and yield strength the ductility is markedly reduced, particularly at 200°C when the elongation can be as low as 15%.
The dependence of weld metal impact strength upon nitrogen content is shown in Table VIII with some supplementary figures for rutile (E307) weld deposits in Table IX. Higher nitrogen contents reduce the impact strengths at all temperatures but the effect is more marked at the lower temperatures. The effect is less noticeable with basic deposits than with acid or rutile deposits, especially at sub-zero temperatures.
3. 3 Effect of nitrogen on the susceptibility of weld metal to microfissuring
Increase in electrode drying temperature reduces the hydrogen content but increases the nitrogen content of weld metal deposited from acid and rutile electrodes (Tables III and IV). The effect of these changes in hydrogen content on weld metil fissuring is shown in Table X and XI. Increase in nitrogen content results in increase in weld fissures despite a simultaneous fall in hydrogen content. The use of supplementary gas shield with acid electrodes,
10
-Table XII, emphasizes these results: welding without shielding gives high nitrogen ^nd hydrogen contents with high fissure contents but shielding with CO„ (to reduce nitrogen and hydrogen content) or H (low nitrogen but very high hydrogen content) eliminated fissures completely (Figs, 3 - 5 ) .
A similar approach of variation in electrode drying temperatures or use of supplementary gas shielding (A, CO , H ) was adopted with British rutile electrodes. The composition of the weld deposits is given in Table XIII. With these electrodes, however, the weld deposits were made in one, two or three runs. The welded samples were then cut into bend test specimens (Fig. 6) which were heat treated for 6 hours at 250°C to remove hydrogen. Bend tests were then made with both dressed and undressed specimens; results are shown in Tible XIV.
Higher nitrogen contents reduced the bend angle at which cracks appeared whilst an increased number of runs also reduced the bend angle.
Metallographic examination of welds deposited by all the electrodes under investigation showed that surface and internal c r a c k s , and microfissures could occur. Surface c r a c k s , which were more prevalent than internal cracks, largely occurred between the weld ripples (Fig. 7) presumably due to the s t r e s s raising effect of the notches. The tendency to cracking increased with nitrogen content and number of weld runs and, in some c a s e s , cracks could be observed visually even without bending of the samples. The extent of microfissuring (Figs. 8, 9) also increased with nitrogen content but it was difficult to obtain a quantitative measure due to their large numbers and difficulties in distinguishing some microfissures from grain boundaries.
3.4 Effect of nitrogen on porosity in weld metal
The possibility of controlling nitrogen in weld deposits from acid, rutile and basic electrodes i s , in general, rather limited. Each of these electrode types has its own characteristic content of nitrogen related to the metallurgical process of welding with their use. Hence the problem of nitrogen effect on porosity in welds has not been investigated more closely so far. Preliminary t e s t s have revealed a fairly pronounced influence of nitrogen content on porosity formation in welds. This susceptibility does not depend solely on the nitrogen content in weld metal and is related m o r e to the total content of nitrogen and hydrogen. The addition of a certain quantity of nitrogenized ferro-alloys to the coverings of rutile and basic electrodes of the mild steel and straight chromium (17% Cr) steel type revealed that at nearly the same hydrogen content in the weld metal, the susceptibility to porosity increased with the increasing content of nitrogen in the weld metal.
The introduction of more than 0.05% nitrogen into the rutile electrode weld metal with a concurrent hydrogen content of more than 10 cm /100 g will give r i s e to severe porosity. The introduction of the same amount of nitrogen into the weld metal from thoroughly dried basic electrodes does not, in general, cause porosity. Similar results were obtained with straight chromium (17% Cr) ferritic steel basic electrodes, although the solubility of nitrogen in steels of this type is much higher than that in mild steels;
nitrogen content in high chromium steels can be up to about 0.2%. The form-ation of porosity in welds which s t a r t s already at the nitrogen and hydrogen content above 0.05% and 4 cm^/100 g respectively also supports the view that there is an interaction of hydrogen and nitrogen due to a decrease in their mutual solubility or to chemical reactions,
A practical case of nitrogen affecting weld porosity occurs when rutile electrodes a r e used. It is commonly known that at the end of melting of a rutile electrode, especially when the current is rather high, weld metal is apt to develop p o r e s . With respect to this feature, rutile electrodes a r e generally made shorter than acid or basic ones, and welding current for rutile electrodes is more restricted at the upper end of the range than is the case with acid electrodes. Rutile electrode covering becomes overheated during welding because at higher temperatures rutile conducts electric current which heats the covering. When welding with relatively long rutile electrodes at high c u r r e n t s , the electrode covering at the end of melting turns bright red. Bringing the covering to such high temperatures gives r i s e to a premature evolution of shielding gases and this in turn can cause an increased nitrogen-ization of weld metal following the appearance of p o r e s .
4. Discussion
The main aim of the work has been to determine the influence of nitrogen on the soundness and properties of weld metal deposited by manual covered electrodes. A subsidiary aim has been to relate the effect of n i t r o -gen to that of hydro-gen in weld metal. Sufficient results have been obtained to show that the nitrogen content of weld metal can be greatly influenced by the choice and prior treatment of electrodes, and to show that nitrogen has a considerable influence on both soundness and properties of weld deposits.
The nitrogen content of mild steel weld metal deposited with covered electrodes is much higher than that found in steel produced by the open hearth steelmaking p r o c e s s . The highest content occurs in rutile weld deposits; it is lower with acid weld deposits and lowest with basic weld deposits. The result of other workers indicate that cellulosic weld deposits can have nitrogen
contents similar to those of basic deposits,
The high nitrogen contents of rutile weld metal is probably due to the large quantities of rutile in the coating and silicon in the weld metal, Rutile is believed to assist the transfer of nitrogen to the weld by the formation of titanium nitride, TiN. Silicon favours the solution of nitrogen in steel so that the higher silicon content (up to 0,35%) in rutile deposits compared to acid deposits (up to 0.1%) could well be a further cause of the higher nitrogen content of rutile deposits.
The relatively low content of nitrogen in basic weld deposits results from the effective gas shield produced by basic electrodes. This gas shield consists largely of carbon dioxide and monoxide from the decomposition of calcium and magnesium carbonates in the coating. Acid and rutils electrodes produce gas shields largely composed of oxides of carbon from carbonate decomposition, together with water vapour and its dissociation products from organic compounds
12
-and water of crystallisation in the coating. However the carbonate content of basic coverings is 20 - 40% (with a CO content of almost half) whereas acid and rutile coverings contain no more than 10% of carbonate. Organic com-pounds in the covering constitute only 3% by weight and make a relatively small contribution to gas shielding, although water in the covering in one form or another makes a larger contribution. The low nitrogen content of cellulosic weld deposits is directly attributed to the copious gas shield from the breakdown of the organic compounds in the covering, consisting of
hydro-gen and oxides of carbon.
It is also necessary to consider the effect of drying electrodes at elevated temperatures before use. The drying of basic electrodes at t e m p e r -atures up to 450 C (to minimise the hydrogen content of the weld metal) has no effect on the quantity of shielding gas available since calcium and magnes-ium carbonates only decompose above 800°C. However, the drying of acid and rutile (and possibly cellulosic) electrodes above 150°C reduces the amount of water available for shielding and hence its efficiency, with a resultant increase in the nitrogen content of the weld deposit (Table III and IV),
The results presented in Tables VI and IX indicate that nitrogen content influences room temperature tensile p r o p e r t i e s , although an anomaly occurs for the British rutile electrodes. The fall in ductility with increase in nitrogen content is particularly serious, with a tensile elongation of 13% and reduction in area of 18% being recorded in one c a s e . Elevated temperatures tensile properties are also affected by nitrogen content (Table VII, figures 1 and 2). There is evidence of the effect of solute atoms on ageing at 200°C and nitrogen would be expected to contribute to this effect. The contribution of nitrogen to strain ageing is also shown in Table VIII.
Nitrogen content also affects the notch impact strength (Tables VIII and IX). The effect is less marked with basic weld deposits and, in general, only becomes really significant for Polish acid and rutile weld deposits at sub-zero t e m p e r a t u r e s . Fewer results were obtained with British rutile electrodes, but notch impact strength at 20°C showed a marked decrease with increased drying temperature and hence increased nitrogen content. This might well be due to the different notches used for the tests reported in Tables VIII and IX. With the British electrodes the impact strength from electrodes deposited in the "as received" condition were typical of rutile deposits. Drying at 200°C had very little effect but drying at 300°C and 400 C resulted in a drastic fall in impact strength.
It is generally accepted that hydrogen is the main cause of microfissures in mild steel weld metal; very few fissures occur in basic low hydrogen weld deposits whilst few fissures a r e also found in acid and rutile weld deposits with hydrogen content below 10 cm^/100 gms of weld metal. In the present work, acid and rutile electrodes have been used with supplementary shielding and after drying at different temperatures in order to establish the effect of nitrogen and hydrogen contents on the extent of fissuring and cracking (Tables X, XI, XII and XIV). With all the electrode coatings examined fissuring increased with increase in nitrogen content, even when the hydrogen content was simultaneously decreased. With the highest nitrogen contents m a c r o -cracking was observed, largely on the surface, whilst the angle of bending at
which cracking was observed was also dependent on nitrogen content. Much more work remains to be done but it is obvious that a re-evaluation of the roles of hydrogen and nitrogen in microfissuring is needed. Again, the
influence of hydrogen on ductility and sensitivity to cracking is only temporary, since hydrogen will diffuse, but nitrogen has a permanent effect.
Nitrogen and hydrogen appear to affect jointly the development of porosity in welds. Again further work is clearly required to establish the exact relationship.
The present work has emphasized the need to keep the nitrogen contents as low as possible to ensure both soundness and ductility. The danger of drying electrodes other than those with basic coverings has also been emphasized by the present work. Although it is not general practice to dry acid or rutile covered electrodes, drying is sometimes carried out, particularly where such electrodes a r e used at the same time as basic electrodes.
5, Conclusions
1. The nitrogen content of weld metal deposited by covered electrodes depends upon the covering type; basic coverings give lower nitrogen contents than acid or rutile coverings.
2. The nitrogen content of acid and rutile weld deposits can be reduced by supplementary gas shielding and increased by drying electrodes at elevated t e m p e r a t u r e s ,
3. Nitrogen increases the yield and tensile strength of mild steel weld metal at room and elevated t e m p e r a t u r e s . An increase in nitrogen content from 0.006% to 0,02% increases the tensile strength from 28 t , s , i . (45kG/mm ) to 35 t . s . i . (55kG/mm^).
4. Nitrogen reduces the ductility and notch toughness of mild steel weld m e t a l s .
5. Nitrogen has a great influence on the incidence of cracking and microfissuring in acid and rutile weld deposits: weld metal with a nitrogen content below 0.007% shows no tendency to cracking or microfissuring even in the presence of high hydrogen contents, e , g , 40 cm /lOO gms.
6. Nitrogen increases the tendency to weld porosity in the presence of hydrogen.
7. The drying of rutile and acid electrodes at temperatures of 200 C and above leads to impaired ductility and impact strength and should be strongly discouraged.
14 -R e f e r e n c e s Sokoiow, E . W . L a k o m s k i , V . J . and G r i g o r e n k o , G . M , B a s z a n o w , K . P . K o b a y a s h i , T , K u w a n a , T , and K i k u c h i , U. W e g r z y n , J . S m i t h e l l s , C , J , S p r a w o c z n i k po S w a r k e , M o s c o w , 1960. Awtom, S w a r k a , 1964, No, 1 1 , Awtog, D e l o , 1956, N o . 4 , Welding in t h e W o r l d , 1967, 5 , 5 8 . P r z e g l a d S p a w a l n i c t w a , 1966, N o s , 6, 7, 8, M e t a l s R e f e r e n c e B o o k , 1955, IIW D o c , I I - A - 1 4 9 - 6 5 , I n t e r n a t i o n a l I n s t i t u t e of Welding, 1 9 6 5 .
TABLE 1
TABLE II
Nitrogen content of weld metal deposited with acid, basic and rutile coatings.
Electrode Type Nitrogen content, % Oxygen content, % Hydrogen content, cm^/100 gms Acid EP52-28 0.012 -0.032 -38 Basic EP49-29 0,012 -Rutile E307 0 , 0 2 3 -0.026 0 . 0 1 3 -0.018 27-32 Rutile EP47-28 0.022 -0.034 -32
Influence of covering thickness of the electrode on the nitrogen content of deposited weld metal.
(Acid electrode, EP52-28, 4 m m . diameter core wire)
Composition of Covering Approx. %
Iron oxides 30, Rutile 20, Carbonates 10, Aluminium silicates 20, Ferromanganese 20, Thickness of Covering, mm. 5 . 4 5 . 8 6 . 2 6 . 5 7 . 0 Nitrogen content of weld deposits % 0.032 0.023 0.018 0.017 0,014
16
-T A B L E III
Influence of e l e c t r o d e d r y i n g t e m p e r a t u r e on t h e
h y d r o g e n and n i t r o g e n c o n t e n t s of d e p o s i t e d weld mietal (Acid e l e c t r o d e E P 5 2 - 2 8 , 4 m m . diameter c o r e wire)
D r y i n g t e m p e r a t u r e of e l e c t r o d e a f t e r e x t r u s i o n °C 20 50 100 200 400 600 800 N i t r o g e n content of weld d e p o s i t % 0 . 0 1 2 0 . 0 1 9 0 . 0 2 3 0 . 0 4 3 0.052 0 . 0 7 1 0 . 0 8 6 Hydrogen content of weld d e p o s i t 3 cm / 1 0 0 g m . weld m e t a l
38 1
30 25 14 12 10 6 T A B L E IV Influence of e l e c t r o d e d r y i n g t e m p e r a t u r e on t h e h y d r o g e n , oxygen and n i t r o g e n c o n t e n t s of deposited weld m e t a l . (Rutile e l e c t r o d e E 3 0 7 , 8 gauge d i a m e t e r ) . D r y i n g t e m p e r a t u r e of e l e c t r o d e1 °^
As r e c e i v e d 200° for 6 h o u r s 2 0 0 ° f o r 12 h o u r s 300° for^ 3 h o u r s 300° for 6 h o u r s 4 0 0 ° f o r 3 h o u r s N i t r o g e n content of weld m e t a l % 0 . 0 2 3 - 0 . 0 2 6 0 . 0 2 7 - 0 . 0 3 0 0 . 0 2 6 - 0 . 0 3 5 0 . 0 2 8 - 0 . 0 3 7 0 . 0 3 2 - 0 . 0 4 3 0 , 0 3 7 - 0 . 0 4 9 Oxygen content of weld m e t a l % 0 . 0 1 3 - 0 . 0 1 8 0 . 0 1 5 - 0 . 0 2 0 0 . 0 1 7 - 0 . 0 2 1 0 . 0 1 8 - 0 . 0 2 4 0 . 0 1 6 - 0 . 0 2 0 0 . 0 2 0 - 0 . 0 2 4 H y d r o g e n content of weld m e t a l 3 cm /lOO g m . weld m e t a l 27-32 2 1 - 2 3 19-22 18-20 14-16 12-15C o m p o s i t i o n of Weld M e t a l % C 0 . 0 7 0 . 0 8 0 . 0 8 Mn 0 . 5 4 0 . 4 6 0 . 4 4 Si 0 . 2 4 0 . 1 9 0 . 2 2 C r 1.1 Mo 0 . 5 4 0 . 5 0 V 0 . 2 4 N i t r o g e n content of weld m e t a l % 0 . 0 2 2 0 . 0 2 8 0 . 0 3 4
TABLE VI
Mechanical properties of weld metal from acid, rutile and basic electrodes deposited in air and in an auxiliary CO shield (15 l i t r e s / m i n . )
Electrode Type Acid EP52-28, air Acid EP52-28,
^°2
Rutile EP47-28, air Rutue EP47-28, ^ ^ 2 Basic EP49-29 air Basic EP49-29 C°2Weld Metal Composition, %
C 0.08 0,08 0.07 0,07 0,07 0,08 Mn 0.57 0,52 0.48 0.50 0.58 0.57 Si t r a c e t r a c e 0,27 0,26 0,46 0,42 P 0.024 0.023 0.018 0.017 0.016 0,018 S 0.021 0.022 0.021 0.020 0.017 0.017 N 0.021 0.006 0.024 0.007 0,012 0,006 Mechanical Properties at 20°C Tensile Strength t , s . i . kG/mm^ 31-36 49-56 26-27 41-43 3 2 , 5 - 51-56 35.5 2 6 . 5 - 42-44 28 2 9 - 46-51 32.5 28-29 44-46 Yield Strength 1 t . s . i . kG/mm^ 26.5 42-47 -30 19- 30-32 20.5 27-31 43-49 19- 30-34 21.5 24-27 38-43 20.5 32-35 -22
E l e c t r o d e T y p e Acid E P 5 2 -1 2 8 , a i r Acid E P 5 2 -2 8 , CO-2 R u t i l e E P 4 7 -2 8 , a i r R u t i l e E P 4 7 -2 8 , CO-2 B a s i c E P 4 9 -29, a i r B a s i c E P 4 9
-1 29, CO2
Units t . s . i . 2 k G / m m t . s . i . 2 k G / m m t . s . i , 2 k G / m m t . s . i . k G / m m ^ t . s . i . k G / m m ' ' t . s . i . k G / m m M e c h a n i c a l P r o p e r t i e s at e l e v a t e d t e m p e r a t u r e s20°C 1
Yield 1 S t r e n g t h 27-30 42-47 1 9 - 2 0 . 5 30-32 27-31 4 3 - 4 9 1 9 - 2 1 , 5 30-34 24-27 3 8 - 4 3 2 0 . 5 - 2 2 . 5 3 2 - 3 5 T e n s i l e S t r e n g t h 31-36 49-56 1 2 6 - 2 7 . 5 4 1 - 4 3 3 2 , 5 - 3 6 51-56 2 6 . 5 - 2 8 4 2 - 4 4 2 9 - 3 2 . 5 46-51 28-29 4 4 - 4 6 200°C Yield S t r e n g t h 2 4 - 2 5 . 5 3 8 - 4 0 1 9 . 5 - 2 1 . 5 3 1 - 3 4 2 3 - 2 4 . 5 3 6 - 3 9 1 9 , 5 - 2 2 , 5 31-35 2 4 - 2 6 . 5 38-42 2 1 , 5 - 2 3 . 5 34-37 T e n s i l e S t r e n g t h 4 0 - 4 2 . 5 6 3 - 6 7 3 3 - 3 4 5 2 - 5 4 4 1 . 5 - 4 5 6 5 - 7 1 3 3 . 5 - 3 6 53-56 3 6 . 5 - 4 0 5 7 - 6 3 3 1 . 5 - 3 5 5 0 - 5 5 300°C Yield S t r e n g t h 1 9 . 5 - 2 3 . 5 31-37 1 8 . 5 - 1 9 . 5 2 9 - 3 1 19-21 3 0 - 3 3 1 9 - 2 0 , 5 30-32 2 0 . 5 - 2 1 3 2 - 3 3 , 5 19-21 3 0 - 3 3 T e n s i l e S t r e n g t h 3 6 . 5 - 3 9 . 5 57-62 2 8 - 3 0 , 5 4 4 - 4 8 3 6 - 3 6 . 5 56-57 2 7 - 3 0 . 5 4 3 - 4 8 3 8 - 4 1 . 5 6 0 - 6 5 28-29 4 4 - 4 6 400°C Yield 1 T e n s i l e S t r e n g t h | S t r e n g t h 1 8 , 5 - 1 9 . 5 29-31 1 9 - 2 3 . 5 30-37 1 6 , 5 - 2 1 2 6 - 3 3 , 5 28-30 44-47 2 6 , 5 - 2 8 , 5 4 2 - 4 5 3 0 - 3 3 . 5 47-53 N i t r o g e n content % 0 . 0 2 1 0 . 0 0 6 0 , 0 2 4 0,007 0 . 0 1 2 0.006Notes: (1) Yield strength was measured at 0,2% offset.
(2) P a r t of the data included in this table is obtained from Brozda, J . , Report No. Z b - 1 , Polish Institute of Welding.
TABLE VIII
Impact strength (Mesnager notch) of weld metal from acid, rutile and basic electrodes deposited in a i r and an auxiliary CO shield (15 l i t r e s / m i n ) .
Electrode Type Acid EP52-28, a i r Acid EP52-28, CO2 Rutile E P 4 7 -28, air Rutile E P 4 7 -28, CO2 Basic EP49-29 a i r Basic EP49-29
^°2
Units ft-lbs 2 kGm/ cm ft-lbs 2 kGm/cm ft-lbs kGm/cm ft -lbs kGm/cm^ ft-lbs kGm/ cm ft-lbs kGm/ cm^ +20°C 58-81 10-14 81-92 14-16 70-87 12-15 81-104 14-18 93-127 16-22 116-139 20-24 0°C 52-81 9-14 75-87 13-15 64-81 11-14 75-93 13-16 87-116 15-20 93-127 16-22 Impact Strength as -20°C 46-69 8-12 64-75 11-13 46-70 8-12 70-87 12-15 75-93 13-16 87-104 15-18 deposited -40°C 18-46 3 - 8 52-64 9-11 24-41 4 - 7 64-81 11-14 58-93 10-16 64-104 11-18 -60°C 6-29 1-5 _ -12-29 2 - 5 29-46 5-8 35-70 6-12 -0 ^Sed +20 C 24-35 4 - 6 71-58 7-10 29-46 5-8 46-64 8-11 58-81 10-14 81-104 14-18 -20°C 6-18 1-3 24-35 4 - 6 12-24 2 - 4 29-41 5-7 35-70 6-12 46-70 8-12 Nitrogen content % 0.021 0.006 0.024 0,007 0.012 0.006Electrode condition As received i Dried 200°C for 6 hours Dried 300°C for 3 hours Dried 400°C for 3 hours Nitrogen Content, % 0.023-0.026 0.027-0.030 0.028-0.037 0.037-0.049 Mechanical P r o p e r t i e s Tensile Strength t . s . i . 3 0 . 5 , 33 30,34 3 5 , 36.5 3 1 , 32.5 kG/mm^ 4 8 . 52 47,57 55,57 49,51 Elongation % 2 4 , 26 22,27 16,19 15,13 Reduction of Area % 6 3 , 68 60,68 35,40 35,18
Charpy V-notch Impact strength at 20°C ft-lbs kGm/cm 86,63.75 75,63,63 26t32t39t l l ! 3 5 : i 7 * 15,11,13 13,11,11 4 . 5 , 5 . 5 , 5 , 0 2 . 6 , 3
T A B L E X
Effect of n i t r o g e n and h y d r o g e n content on weld m e t a l s o u n d n e s s f o r acid weld d e p o s i t s . E l e c t r o d e d r y i n g t e m p e r a t u r e °C 50 100 200 400 600 800 Weld m e t a l c o m p o s i t i o n , % C 0.06 0 . 0 8 0 . 0 8 0.07 0 , 0 8 0.07 Mn 0.47 0.49 0 . 4 8 0 . 5 2 0.47 0 . 4 3 Si 0 . 0 6 0 . 0 8 0 . 0 8 0.07 0 . 0 6 0 . 0 4 P 0.027 0 . 0 3 0 0 . 0 3 1 0.027 0.027 0 , 0 2 6 S 0 . 0 2 0 0 , 0 2 3 0 , 0 1 9 0 . 0 1 9 0 . 0 2 0 0.021 N 0 , 0 1 9 0 . 0 2 3 0 . 0 4 3 0 , 0 5 2 0 . 0 7 1 0 . 0 8 6 H y d r o g e n c o n t e n t . c m ^ / 1 0 0 gm weld m e t a l 30 25 14 12 10 6 N u m b e r of m i c r o f i s s u r e s a f t e r cooling in a i r . Up to 8 m i c r o f i s s u r e s Up to 4 m i c r o f i s s u r e s Up to 8 m i c r o f i s s u r e s Up to 10 m i c r o f i s s u r e s M i c r o & m a c r o f i s s u r e s , p o r e s M i c r o & m a c r o f i s s u r e s , p o r e s to to
s o u n d n e s s for r u t i l e weld d e p o s i t s . E l e c t r o d e d r y i n g t e m p e r a t u r e °C 50 100 200 400 600 800
1
Weld m e t a l c o m p o s i t i o n , %c
0 , 0 5 0 . 0 7 0 . 0 8 0 . 0 8 0 . 0 7 0.07 Mn 0 . 4 3 0 . 4 7 0 . 4 5 0 . 5 9 0 . 5 8 0 . 5 8 Si 0 . 1 4 0 . 1 7 0 . 1 6 0 . 2 3 0 . 2 4 0 . 2 2 P 0 . 0 2 0 0 . 0 2 2 0 . 0 2 0 0 . 0 2 3 0 . 0 2 1 0 . 0 2 2 S 0 . 0 2 0 0 . 0 2 3 0.019 0 . 0 2 0 0 . 0 2 1 0 . 0 2 0 N 0 . 0 2 2 0 . 0 2 7 0 . 0 4 3 0 . 0 5 1 0 . 0 7 6 0.092 H y d r o g e n c o n t e n t , c m ^ / 1 0 0 gm weld m e t a l 32 26 19 16 14 5 N u m b e r of m i c r o f i s s u r e s a f t e r cooling in a i r Up t o 5 m i c r o f i s s u r e s Up to 5 m i c r o f i s s u r e s Up to 6 m i c r o f i s s u r e s M i c r o and m a c r o f i s s u r e s , p o r e s M i c r o and m a c r o f i s s u r e s , p o r e s M a c r o f i s s u r e s , p o r e sTABLE X n
Effect of nitrogen and hydrogen content on the tendency to microfissuring of acid (EP52-28) weld deposits.
Welding Conditions
Weld deposited in air
Weld deposited in
COg gas shield (15 litres/min)
Weld deposited in Hydrogen gas shield (10 litres/min) Nitrogen Content % 0.024 0.006 0.006 Hydrogen content cm^/100 gms weld metal 28 20 40 Number of microfissures after cooling in air
More than 15
No microfissures
Electrode and welding conditions
As received
Dried 200°C for 6 hours
Dried 200°C for 12 hours
Dried SOofc for 3 hours
Dried 300°C for 6 hours
Dried 4C0'^C for 3 hours
Dried 400°C for 3 hours deposited in argon shield (7 l i t r e s / m i n )
Dried 400°C for 3 hours deposited in CO_ shield (7 l i t r e s / m i n )
Dried 400°C for 3 hours deposited in H shield (7 l i t r e s / m i n ) C 0.06 0.06 0.07 0.06 0.06 0.06 0.06 0.07 0.07 Composition Mn 0.69 0.67 0.65 0.61 0.58 0,54 0,58 0.52 0.55 Si 0.17 0.18 0.16 0.15 0.15 0.16 0,18 0.14 0.15 . % P 0.028 0.027 0.025 0.027 0.024 0.026 0.025 0.026 0.026 S 0.020 0.022 0.019 0,020 0.018 0.019 0.019 0.020 0,019 N 0.023-0.026 0.026-0.030 0.026-0.035 0.028-0.037 0.032-0.043 0.037-0.049 0.012-0.014 0.013-0,015 0,012-0.014 O 0 . 0 1 3 -0.018 0.015-0.020 0.017-0.021 0.018-0.024 0.016-0.020 0.020-0,024 0,018-0,020 0.019-0.022 0.018-0.020 Hydrogen content cm /100 gm weld metal 27 - 32 2 1 - 2 3 1 9 - 2 2 1 8 - 2 0 1 4 - 1 6 1 2 - 1 5 1 3 - 1 6 1 4 - 1 5 3 7 - 4 0
1
TABLE XIV
Correlation between nitrogen content of rutile (E307) weld metal, number of runs and bend ductility.
Nitrogen Content, % 0.023-0.026 0.028-0.037 0.037-0,049
Angle of bending at which cracks appeared
1 run deposit 180°C, no cracks 150° 60° 2 run deposit 120° 50° 30° 3 run deposit 100° 30° 15° to
100 150 200 250 TEMPERATURE (°C) -LEGEND 1. TENSILE STRENGTH
2. YIELD STRENGTH 3. TENSILE EUONGAFION
FIG. 1. ELE\«\TED TEMPERATURE PROPERTES OF ADD (EP 5 2 / 2 8 ) WELD DEPOSITS.
Q
g
_ i ai 2 te 50 100 150 200 250 TEMPERATURE PC) • 300 350 LEGEND 1 TENSILE STRENGTH.2 YIELD STRENGTH, a TENSILE ELONGATION
FIG. 2. ELEVATED TEMPERATURE PROPERTIES OF BASIC ( E P ^ 9 - 2 9 ) WELD DEPOSITS.
F I G . 3 B E A D - O N - P L A T E BEND TEST SPECIMEN D E P O S I T E D BY AN ACID E L E C T R O D E . HYDROGEN C O N T E N T 28 c m 3 / 1 0 0 g m ; NITROGEN C O N T E N T 0. 024%; 15 MICROCRACKS F I G . 4 B E A D - O N - P L A T E BEND T E S T S P E C I M E N D E P O S I T E D BY ACID E L E C T R O D E IN A CO„ A T M O S P H E R E . HYDROGEN C O N T E N T 20 cm / 1 0 0 g m ; NITROGEN C O N T E N T 0. 006%; NO MICROCRACKS F I G . 5 B E A D - O N - P L A T E BEND T E S T SPECIMEN D E P O S I T E D BY AN ACID E L E C T R O D E IN AN H „ 0 A T M O S P H E R E . ^ HYDROGEN CONTENT 40 c m 3 / 1 0 0 g m ; NITROGEN CONTENT 0. 006%; NO MICROCRACKS
- ^ \ 5 mm-•FOR 3 RUNS)
F I G . 7 CRACKS BETWEEN WELD R I P P L E S ; F R O M A B E A D O N -P L A T E D E -P O S I T (3 RUNS) WITH A NITROGEN C O N T E N T O F 0 . 040% X 2. 5 F I G . 8 B E A D - O N - P L A T E BEND T E S T SPECIMEN D E P O S I T E D BY A RUTILE E L E C T R O D E . T O P : NITROGEN C O N T E N T 0. 040% HYDROGEN C O N T E N T 15 c m ^ / l O O g m . B O T T O M : NITROGEN C O N T E N T 0. 013% HYDROGEN C O N T E N T 15 c m ^ / 1 0 0 g m .
