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C R A N F I E L D
I N S T I T U T E OF T E C H N O L O G Y
THE EFFECT OF HEAT INPUT AND POST-WELD HEAT TREATMENT
ON MILD AND LOW ALLOY STEEL SUBMERGED ARC WELD DEPOSITS
by
August 1972.
THE EFFECT OF HEAT INPUT AND POST-WELD HEAT TREATMENT ON MILU AND LOW ALLOY STEEL SUBMERGED ARC WELD DEPOSITS
by D.J. Campbell, B.Sc, R.L. Apps, Ph.D., B.Sc, F.I.M., M.Inst.W. and E. Smith, Ph.D., B.Sc, A.I.M. SUMMARY
Submerged arc welds were prepared in quenched and tempered steel plates using a mild steel wire, two low alloy steel wires, and commercially available fluxes. Heat inputs of 1.2, 2.2, and 4.3 KJ/mm were used for each wire/flux combination and in each case the preheat/ interpass temperature was maintained within the range 120 - 150 C. Sections from each weld were given post-weld heat treatments at 550 C and 650 C for 1 hour followed by air cooling. The weld metal properties were evaluated using Charpy, tensile, and hardness tests and structures were examined using optical and electron microscopy. The impact properties of the mild steel deposit were unaffected by
variations in heat input and by post-weld heat treatment but strength generally decreased with increasing heat input and after post-weld heat
treatment. In one of the low alloy steel weld deposits the best impact properties were achieved at the lowest heat input but strength and hardness were unaffected by heat input variations. Post-weld heat treatment improved the impact properties at all heat input levels but reduced strength and hardness. The other low alloy steel weld deposit gave the best impact properties at the medium and high heat input levels but strength and hardness were generally unaffected by variations in heat input. Post-weld heat treatment, however, raised the transition temperature and increased the strength and hardness at all heat input levels. This was associated with the presence of vanadium in the welding wire.
PAGE
1. INTRODUCTION 1
2. MATERIALS 1
3. PROCEDURE 2
4. RESULTS 3 4.1 General welding characteristics 3
4.2 Chemical analyses 4 4.3 Impact properties 4 4.4 Tensile and hardness properties 5
4.5 Metallography 5 5. DISCUSSION 8 6. CONCLUSIONS 11 7. ACKNOWLEDGEMENT 12 REFERENCES 13 TABLES
1. Compositions of plate materials 2 2. Compositions of welding wires 2
3. Compositions of fluxes 2 4. Details of welding conditions 3
5. Chemical analyses of weld deposits 6 6. Mechanical properties of weld deposits 7 7. Inclusion counts on weld deposits 6
Location of test pieces.
Charpy transition curves for Bostrand MS65/Encrex weld deposits - region of minimum dilution.
Charpy transition curves for Bostrand MS65/Encrex weld deposits - region of maximum dilution.
Charpy transition curves for SD3/OP41TT weld deposits -region of minimum dilution.
Charpy transition curves for SD3/OP41TT weld deposits -region of maximum dilution.
Charpy transition curves for Bostrand 31/Encrex weld deposits region of minimum dilution.
Charpy transition curves for Bostrand 31/Encrex weld deposits region of maximum dilution.
Effect of cooling rate after tempering at 650 C on Charpy transition curves of Bostrand 31/Encrex welds deposited at 2.2 KJ/mtn - region of minimum dilution.
Effect of prolonged heat treatment at 150 C for Bostrand 31/ Encrex weld deposits - region of minimum dilution.
1. INTRODUCTION
Increasing interest in the use of quenched and tempered steels has shown that considerable impairment of toughness, as measured by the Charpy V-notch impact test, occurs in regions of the heat affected
zone (HAZ) exposed to temperatures above the critical range during submerged arc welding, accompanied by considerable increases in strength and hardnesses frequently in excess of 400 HV5 . In QT35, HAZ toughness decreased further when the heat input was raised above the recommended range of 1.2 - 2.6 KJ/mm due to a change in microstructure from autotempered martensite to upper
bainite. This work has been substantiated by crack opening displacement tests on HAZs in QT35^ and by Charpy tests on simulated HAZs in T-1 steel^»^.
In contrast the HAZ toughness of HY80 and Navy Ql steels is not degraded further by heat inputs above the recommended range of 1.2 - 2.2 KJ/mm;
in fact toughness improved slightly with increasing heat input up to 4.3 KJ/ mm-^' . Other published work has concluded that HAZ toughness, as measured
by the crack opening displacement test, is little affected by variations in heat input up to 2.7 KJ/mm in HY80 steel.^ This has been attributed to the fact that the HAZ structure in HY80 and Navy Ql steels remains essentially martensitie even with comparatively high heat inputs.^
Post-weld heat treatments for 1 hour at 550 C and 650 C have been shown to be very effective in improving toughness and reducing strength and hardness in the HAZ of QT35, HY80, and Navy Ql steels.2-5 in this
respect treatment at 650 C was better and brought the impact properties close to parent material levels.
From the point of view of the HAZ alone there are, therefore, two recognisable benefits to be derived from modifications to existing welding procedures. First an economic benefit from the use of higher heat input since this would reduce the total time spent in welding a given joint; and secondly a benefit in terms of improved toughness by the use of post-weld heat treatment. The first of these benefits is perhaps the most attractive since there are no obvious concomitant disadvantages. The second benefit is open to question because of the obvious economic penalty associated with the use of post-weld heat treatment.
However before either of these modifications to existing welding procedures can be seriously considered it is necessary to establish that they will not adversely affect the properties of the weld metal since at least one worker has shown that the weld metal is more important than the HAZ in
determining the overall toughness of the welded joint.9
The main object of the present work was to examine the structure and properties of submerged arc weld deposits prepared with commercially available consumables recommended for quenched and tempered steels. The effect of heat
input variations in the range 1.2 - 4.3 KJ/mm and of post-weld heat treatments at 550 C and 650 C were assessed. The effect of prolonging the preheat/ interpass temperature for periods up to one month after welding was examined for one of the weld deposits because of reported instances of this occurring in practice.^
2. MATERIALS
The compositions of the 38mm quenched and tempered steel plates used for the test welds are given in table 1. Details of the welding wires and fluxes are shown in tables 2 and 3 respectively. The two fluxes were of the agglomerated type and may be described as semibasic (Encrex 2/102) and fully basic (0P41TT). The SD3/INi ^Mo wire was appreciably higher in C, Mn, and Ni than the specification (C 0.08 - 0.12, Mn 1.3 - 1.6, Ni 0.9 - 1.1).
Material QT35 HY80 Navy Ql
C
0.14 0.17 0.14 Mn 0.97 0.33 0.35 Si 0.15 0.28 0.25S
0.033 0.015 0.009P
0.010 0.009 0.004 Ni 1.10 2.08 2.40 Cr 0.78 1.41 1.28 Mo 0.41 0.24 0.32V
0.040 0.003 0.010 Cu 0.07 0.12 0.08 Ti 0.001Table 1: Compositions of plate materials,
Wire Type Bostrand MS65 SD3/IN.
iMo
Bostrand 31 o •l-l u n) u Oz
A
B
C
. •r-l Q 1.6 3.2 1.6c
0.09 0.14 0.07 Mn 1.16 1.83 1.41 Si 0.62 0.23 0.45S
0.007 0.008 0.006P
0.016 0.015 0.011 Ni 0.14 1.37 1.31 Cr 0.06 0.10 0.05 Mo <0.05 0.56 0.40V
<J).05 0.05 0.16C
0.21 0.23 0.05 Al (tot) 0.090 0.003 0.005Table 2: Compositions of welding wires.
Flux Type Encrex 2/102 0P41TT Nota-tion X Y Si02 21 9 CaO+ MgO 40 36 CaF^ 11 30 TiO^ 3.5 Na^O 1.5 AI2O3 25 H2O 0.35 Basi-* city 2.3 3.1
* calculated according to British Patent 1, 021, 923 Table 3: Compositions of fluxes.
11
3. PROCEDURE
Test welds were prepared by joining two plates 460mm x 300mm along the 460mm length using the preparation illustrated in fig. 1 and the conditions shown in table 4. Test welds will be referred to by the notation used in table 4 in which the first letter refers to the wire (table 2)
and the second letter refers to the flux (table 3). The root face was increased from 3.2mm to 6.4mm for the BY series of test welds because of the more deeply penetrating characteristics of the larger diameter wire, A flat
characteristic transformer-rectifier and constant feed speed unit was used for test welds CX/1 - CX/3; the remainder were prepared using a voltage controlled wire-feed system and drooping characteristic power source.
The test plate assembly was preheated to 120°C and the root run was deposited in the larger Vee. The heat input for this run was not allowed to exceed 2.2 KJ/ mm and 1.2 KJ/mm respectively for the smaller and larger diameter wires.
After deslagging a second weld pass was deposited on top of the root run at the nominal heat input. The test plate assembly was then turned over and the weld preparation ground back to weld metal. Welding of the smaller Vee was then completed at the nominal heat input and a final tempering bead was deposited. The test plate assembly was then turned over again and welding of the larger Vee was completed at the nominal heat input and a final tempering bead was deposited. An interpass temperature in the range 120 - 150°C was maintained during welding.
Wel d Notatio n AX/1 AX/2 AX/3 BY/1 BY/2 BY/3 CX/1 CX/2 CX/3 t-i cd (U-H (0 M cd lu CQ U cd Navy Ql Navy Ql HYbO Navy Ql Navy Ql Navy Ql Navy Ql Navy Ql Navy Ql QT35 Wire Bostrand MS65 Bostrand MS65 Bostrand MS65 SD3/INi iMo SD3/INi iMo SD3/INi iMo Bostrand 31 Bostrand 31 Bostrand 31 Boxtrand 31 Flux Encrex 2/102 Encrex 2/102 Encrex 2/102 0P41TT 0P41TT 0P41TT Encrex 2/102 Encrex 2/102 Encrex 2/102 Encrex 2/102
Root run conditions Amps 245 200 200 400 350 350 200 230 315 200 Volts 32 30 30 30 30 30 31 31 31 30 Speed mm/ sec 6.30 2.75 2.75 10.15 13.55 13.55 4.65 3.40 4.65 2.75 Heat in-put KJ/ mm 1.3 2.2 2.2 1.2 0.8 0.8 1.3 2.1 2.1 2.2 Conditions after root run 1 Amps 265 200 230 400 400 I 500 225 235 300 200 Volts 29 30 29 30 30 30 31 31 31 30 Speed] mm/ sec 6.70 2.75 1.55 10.15 5.50 3.40 5.90 3.40 2.10 2.75 Heat in-put KJ/ ram 1.1 2.2 4.3 1.2 2.2 4.3 1.2 2.1 4.4 2.2 Tota l passe s 35 15 13 30 1 17 10 28 14 10 15
Table 4: Details of welding conditions.
The completed test welds were sectioned transversely into 12mm slices. Some of these were retained in the 'as welded' condition. In all test welds except CX/4 the remaining slices were heat treated at 550 C or 650 C for 1 hour followed by air cooling in order to simulate post-weld heat treatment. In test weld CX/4
the remaining slices were given either a simulated post-weld heat treatment at 650 C for 1 hour followed by water quenching or a heat treatment at 150 C for periods of 1 day, 1 week, and 1 month followed by air cooling in order to study the effect of maintaining the preheat/interpass temperature after welding.
lOmm square Charpy V-notch specimens and tensile test pieces with a gauge length of 7.6ram and a diameter of 4.5ram were prepared from the regions of minimum and maximum dilution of the welds as shown in fig. 2. The tensile tests were carried out on an Instron testing machine at a strain rate of
approximately 3 x 10~3/sec. Hardness tests were carried out using a Zwick
hardness tester with a load of 5 Kgm. Weld metal structures were examined using optical microscopy and electron microscopy of carbon extraction replicas.
Inclusion counts on weld metals were made by counting the number of inclusions traversed in a known area at a magnification of 500x. Chemical analyses were carried out on samples taken from the broken Charpy specimens.
4. RESULTS
4.1. General welding characteristics
smooth with only small variation in ripple. Slag detachability was
generally good except in the root runs where extensive chipping and grinding was necessary. Flux Y gave more difficulties in this respect than flux X.
Similar difficulties have been reported for submerged arc welding with low SiO„ fluxes.12
4.2. Chemical Analyses
Chemical analyses for the minimum and maximum dilution regions of the test welds are shown in table 5. Variation in heat input had little effect on the composition of the weld metals except for a slight increase in the Mn and Si contents of the AX and CX series with increasing heat input. The maximum dilution regions were generally higher in C, Ni, and Cr and lower in Mn than the minimum dilution regions due to the greater degree of dilution by the parent material experienced by the former. The exceptionally low Si contents in both regions of test weld AX/2 and the high Ni content in the maximum dilution region of the same weld are probably sampling errors rather than true variations in weld metal composition. The analyses are generally closely related to the wire and flux compositions shown in tables 2 and 3 but with the C, Mn, and Si contents lower than in the corresponding wires. The BY series was significantly higher in Al than the wire due to the high A1„0_ content of the flux. The AX series contained mainly Mn and Si. The BY and CX series were characterised by an approximate composition of 1% Mn, 1.5% Ni, and 0.5% Mo in the minimum dilution region. However, the BY series was generally higher in C, Mn, Ni, Cr, Mo, Cu, and Al and lower in Si and V than the CX series. The BY series had the lowest Si contents as would be expected from the low Si contents of the consumables.
4.3. Impact Properties
The Charpy transition curves are shown in figs. 3 - 1 0 . The impact properties of series AX was not significantly affected by either variation in heat input or post-weld heat treatment (figs. 3 and 4 ) .
Typical 55J (40ft.Ibf) temperatures for the region of minimum dilution were in the range - 30 C to -40 C with upper shelf energies in the range 160 - 190J. The 55J temperature was generally 15 C to 45 C higher in the region of
maximum dilution coupled with a significantly reduced upper shelf energy. The BY series exhibited the lowest transition temperatures with the 55J temperature in the range -40 C to -55 C for the region of minimum dilution (fig. 5). The lowest transition temperature was recorded for weld metal BY/1 which was deposited at the lowest heat input (1.2 KJ/mm). Post-weld heat treatment at 550 C had little effect on impact performance except for a small reduction in energy absorption at low temperatures. Post-weld heat treatment at 650 C significantly improved impact performance at all temperatures with a lowering of the 55J temperature by 15 C to 20 C and an increase in the upper shelf energy from lOOJ to 120-140J. In the region of maximum dilution the 55J temperature was some 25 - 30 C lower than in the
region of minimum dilution although the upper shelf energies were very similar (fig. 6).
The impact properties of series CX were generally better at the 2.2 and 4.3 KJ/mm heat input levels than at 1.2 KJ/mm heat input (figs, 7 and 8 ) . Transition temperatures were higher than for the BY series with typical 55J temperatures in the range -20°C to -30°C for the region of minimum dilution. Post-weld heat treatments at 550 C and 650°C raised the transition temperatures by approximately 15 C and 30 C respectively. The maximum dilution region was characterised by transition temperatures 10°C to 35°C higher than in the
minimum dilution region. Water quenching after tempering at 650°C further raised the transition temperature of the minimum dilution region of test weld CX/4 by about 10 C and reduced the upper shelf energy, fig. 9. The continued application of the preheat/interpass temperature at 150 C for periods
of up to one month after welding did not impair the impact properties of weld metal CX/4 in the minimum dilution region, as shown in fig. 10. In fact the
treatment produced a slight improvement in impact performance in the transition range.
4.4 Tensile and hardness properties
The results of tensile and hardness tests are given in table 6. The strength of series AX decreased with increasing heat input and generally decreased after post-weld heat treatment. Hardness showed little variation with heat input but generally decreased slightly after post-weld heat
treatment. The tensile and hardness properties of series BY and CX were not significantly affected by variations in heat input. In general the BY series had higher strength and hardness than the CX series. Post-weld heat
treatment at 550 C had little effect on the tensile and hardness properties of the BY series. Post-weld heat treatment at 650 C reduced the proof stress and the UTS by about 10% and the hardness by about 40 HV5. In contrast
post-weld heat treatment at 550 C and 650°C produced small increases in proof stress and UTS of up to 10% in the CX series and corresponding increases in hardness of about 10 HV5. An increase in hardness of 21 HV5 occurred when the post-weld heat treatment at 650 C was followed by water quenching. The continued application of the preheat/interpass temperature had no effect on the tensile and hardness properties of the CX series.
4.5 Metallography
The results of the inclusion counts are given in table 7. There is a general decrease in inclusion content with increasing heat input for each series which is probably due to the reduction in the number of passes
accompanying increases in heat input. The BY series had the lowest inclusion contents which is consistent with its low Si content. Typical microstructures of series AX, BY, and CX are shown in figs. 11 - 13. The most significant difference is in the amount of proeutectoid ferrite present. Series AX contained relatively large areas of this product (fig. 11a) and had a much coarser structure than the BY and the CX series. The BY series was
extremely fine grained and contained little proeutectoid ferrite. The CX series contained a small amount of proeutectoid ferrite and was generally coarser than the BY series. The predominant constituent in the BY and CX series was fine acicular ferrite. Post-weld heat treatment at 550 C produced little change in microstructure but post-weld heat treatment at 650 C brought about carbide precipitation in all weld metals. There was less carbide precipitation in the CX series which is probably a reflection of its lower carbon content.
Weld No. AX/1 AX/2 AX/3 BY/1 BY/2 BY/3 CX/1 CX/2 CX/3 R e g i o n of weld d e p o s i t Minimum d i l u t i o n Maximum d i l u t i o n Minimum d i l u t i o n Maximum d i l u t i o n Minimum d i l u t i o n Maximum d i l u t i o n Minimum d i l u t i o n Maximum d i l u t i o n Minimum d i l u t i o n Maximum d i l u t i o n Minimum d i l u t i o n Minimum d i l u t i o n Maximum d i l u t i o n Minimum d i l u t i o n Maximum d i l u t i o n Minimum d i l u t i o n Maximum d i l u t i o n C h e m i c a l A n a l y s i s C 0 . 0 8 0 . 1 1 0 . 0 8 0 . 1 0 0 . 0 8 0 . 1 2 0 . 0 9 0 . 1 2 0 . 0 8 0 . 1 2 0 . 0 9 0 . 0 6 0 . 0 9 0 . 0 7 0 . 0 9 0 . 0 6 0 . 0 8 Mn 0 . 7 2 0 . 6 2 0 . 7 7 0 . 7 2 0 . 9 2 0 . 7 7 1.24 1 . 1 2 1.27 1 . 0 1 1 . 2 3 0 . 7 9 0 . 7 1 0 . 9 4 0 . 7 5 1 . 1 3 1 . 0 2 S i 0 . 4 6 0 . 3 9 0 . 1 1 0 . 1 0 0 . 5 4 0 . 4 9 0 . 1 9 0 . 2 5 0 . 1 9 0 . 2 0 0 . 2 3 0 . 2 3 0 . 2 4 0 . 2 9 0 . 2 3 0 . 3 2 0 . 3 3 S 0 . 0 0 5 0 . 0 0 7 0 . 0 0 5 0 . 0 0 5 0 . 0 0 1 0 . 0 0 5 0 . 0 1 1 0 . 0 1 1 0 . 0 0 8 0 . 0 0 9 0 . 0 0 8 0 . 0 0 6 0 . 0 0 7 0 . 0 0 5 0 . 0 0 7 0 . 0 0 5 0 . 0 0 6 P 0 . 0 1 6 0 . 0 1 5 0 . 0 1 7 0 . 0 1 6 0 . 0 1 6 0 . 0 1 6 0 . 0 1 9 0 . 0 1 5 0 . 0 1 9 0 . 0 1 6 0 . 0 1 8 0 . 0 1 1 0 . 0 1 0 0 . 0 1 2 0 . 0 1 0 0 . 0 1 3 0 . 0 1 1 Ni 0 . 2 8 0 , 6 3 0 . 4 1 1 . 7 6 0 . 3 0 0 . 6 1 1 . 4 6 1 . 6 5 1.49 1.75 1.55 1.37 1 . 6 1 1 . 4 2 1 . 6 8 1 . 3 5 1 . 5 4 Cr 0 . 1 9 0 . 4 3 0 . 2 3 0 . 4 0 0 . 2 0 0 . 4 2 0 . 2 1 0 . 4 4 0 . 2 4 0 . 5 6 0 . 3 3 0 . 1 2 0 . 3 3 0 . 1 2 0 . 4 2 0 . 0 4 0.3?. Mo < 0 . 0 5 0 . 0 6 < 0 . 0 5 0 . 1 0 < 0 . 0 5 < 0 . 0 5 0 . 5 8 0 . 5 2 0 . 5 6 0 . 5 1 0 . 5 5 0 . 4 1 0 . 3 8 0 . 4 1 0 . 3 9 0 . 4 2 0 . 3 9 V < b . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5
<::o.o5
0 . 1 2 0 . 1 0 0 . 1 2 0 . 0 9 0 . 1 5 0 . 1 2 Cu 0 . 2 1 0 . 1 9 0 . 1 9 0 . 1 6 0 . 2 1 0 . 1 9ó.ii
0 . 1 7 0 . 2 2 0 . 1 5 0 . 2 0 < J ) . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 < 0 . 0 5 ' Al ( t o t -a l ) 0 . 0 1 0 0 . 0 1 7 0 . 0 1 1 0 . 0 0 9 0 . 0 1 2 0 . 0 1 4 0 . 0 1 7 0 . 0 2 0 0 . 0 1 9 0 . 0 2 0 0 . 0 1 6 0 . 0 0 4 0 . 0 0 4 0 . 0 0 8 0 . 0 0 5 0 . 0 0 5 0 . 0 0 4Table 5: Chemical analyses of weld deposits.
Weld N o . AX/1 AX/2 AX/3 A v e r a g e I n c l u s i o n s 2 p e r mm 1 5 . 6 0 0 1 4 , 7 0 0 1 1 , 9 0 0 1 4 , 1 0 0 Weld No. BY/1 BY/2 BY/3 A v e r a g e I n c l u s i o n s 2 p e r mm 1 5 , 6 0 0 9 , 6 0 0 1 0 , 4 0 0 1 1 , 9 0 0 1 Weld N o . CX/1 CX/2 CX/3 A v e r a g e I n c l u s i o n s 2 p e r mm 2 2 , 7 0 0 1 8 , 8 0 0 1 6 , 7 0 0 1 9 , 4 0 0
Weld
No.
AX/1 AX/2 AX/3 BY/1 BY/2 BY/3 CX/1 CX/2 CX/3 CX/4 Post-weld heat treat-ment None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ih at 550°C Ih at 650°C None Ihat 650°C* 24hat 150°C 168hat 150°C 672hat 150°C Minimum d] 0.2%proof stress N/mm (tonf/, in ) 497 (32.2) 429 (27.8) 442 (28.6) 440 (28.5) -388 (25.1) 414 (26.8) 428 (27.7) 772 (50.0) -644 (41.7) 680 (44.0) 710 (46.0) 636 (41.2) 772 (50.0) -670 (43.4) 652 (42.2) -705 (45.7) 698 (45.2) 733 (47.5) 707 (45.8) 659 (42.7) -718 (46.5) 656 (42.5) 710 (46.0) 645 (41.8) 655 (42.4) 639 (41.4) Llution ; UTS ... 2 _(?6^/in^)
590
519
525
548
514
539
550
787
702
766
764
684
821
760
692
763
752
764
746
710
760
lie
780
730
730
715
Oè.ï)
(33.6) (34.0) (35.5) -(33.3) (34.9) (35.6) (51.0) -(45.5) (49.6) (49.5) (44.3) (53.2) -(49.2) (44.8) -(49.4) (48.7) (49.5) (48.3) (46.0) -(49.2) (47.8) (50.5) (47.3) (47.3) (46.3) zone Reduc-tionin
Area (%)n
75
74
n
-75
75
78
66
-70
67
68
71
60
-67
66
-66
66
68
65
66
-66
69
70
71
72
71
Hard-nessHV5
206
197
197
188
178
177
205
196
203
268
263
237
271
261
224
266
262
231
236
244
252
238
250
253
241
245
253
i^6
277
253
253
252
Maximum dilution zone 0.2%proof stress in) 565 (36.6) 520 (33.7) 508 (32.9) -448 (29.0) 446 (28.9) 442 (28.6) -787 (51.0) 792 (51.3) 698 (45.2) " • -772 (50.0) 787 (51.0) 719 (46.6)
-UTS
_ . / ^ n(te/in^)
6Si (42,3)
615 (39.8) 599 (38,8) -560 (36.3) 560 (36.3) 551 (35.7) -800 (51.8) 815 (52.8) 738 (47.8) ~" -787 (51.0) 809 (52.4) 790 (51.2) - Reduc-tionin
Area (%)71
67
72
-73
76
70
-60
68
66
• •-67
65
66
-> _ Hard-nessHV5
111
210
211
231
224
111211
107219
294
294
258
289
279
252
264
258
226
246
261
255
261
270
266
261
267
265
-water quenched after tempering.
5. DISCUSSION
The welding characteristics important in producing fracture tough weld metals by the submerged arc process will depend to a great extent on the compositions of the filler wire and flux and may not be the same as those that produce good fracture toughness in the HAZ. A particularly difficult problem is to secure weld metal comparable in toughness to a quenched and tempered steel. In many cases, therefore, weld metals have been designed to achieve adequate toughness without necessarily matching the properties of the base metal. It is important to realise that improvement in weld metal
toughness must not be achieved at the expense of strength. In the welding of mild steel there is some protection against brittle fracture because of the pronounced over-matching in strength displayed by most weld metals with existing consumables.13 In higher strength steels, however, the weld metal may be the more critical part of the welded joint since under-matching weld metals may require greater fracture toughness than base metal if the full properties of the base metal are to be realised.'» 13
The most useful comparison which can be made from the present results is between the BY and the CX series of weld metals since these
represent two sets of commercially available consumables which have been recommended for welding quenched and tempered steels of the QT35 and HY80 types. The BY series had lower transition temperatures, lower maximum energy absorptions, and higher strengths than the CX series over the heat input range used. It is not possible to ascribe these effects to any single factor. In fact there are a number of factors which may account for this.
It has been shown that increasing flux basicity favours lower transition temperatures by producing weld metals of lower inclusion and oxygen contents.12» 14 - 19 xhe present results followed this trend with the flux for the BY series having a basicity of 3.1 compared with a value of 2.3 for the flux used for the CX series, calculated according to the formula given in British Patent 1, 021, 923.11
Weld metal composition also has a profound influence on mechanical properties. The analyses of the BY and the CX series were
generally similar but small variations did exist which would account for the observed differences in mechanical properties. For example the higher Mn, Ni, Cu and Al and the lower Si and V contents of the BY series would promote
lower transition temperatures than in the CX series although this effect would be counterbalanced to some extent by the higher C, Cr, and P contents of the BY series.20 - 25 xhe higher C and Ni contents of the BY series may also explain its lower maximum energy absorption.^ • In addition TULIANI et all2 have recently shown that the maximum energy absorption depends upon the Mn/Si ratio with ratios between 2 and 4 giving the highest values. The present results are in agreement with this since the Mn/Si ratios were 6.2 and 3.4 respectively for the BY and the CX series.
Variations in other chemical elements, however, are likely to affect this relationship.22. 2 .j.^^^ higher alloy content of the BY series would also explain its higher proof stress, tensile strength and hardness compared with the CX series. It should be remembered, however, that the wire used for the BY series was appreciably higher in C, Mn and Ni than its specification.
It has been suggested that notch impact strength can be significantly improved by reducing the inclusion and oxygen contents of the weld metal.1' In the present work the BY series had a lower inclusion content than the CX series but no oxygen determinations were carried out. Inclusions in the weld metal would be expected to affect the ductile rather than the brittle mode of fracture and it is the feeling of the present authors that a reduction in oxygen content is far more beneficial to fracture toughness than a reduction in inclusion content. The observed benefit to fracture toughness of a low inclusion content may be due, in reality, to the fact that a low inclusion content is generally associated with a low oxygen content.
The grain size of the BY series was generally finer than that of the CX series which may be another factor contributing to the superior toughness of the former. The smaller proportion of proeutectoid ferrite
in the microstructure of the BY series may also contribute. This may be due to the higher alloy content of the BY series which would tend to suppress the
formation of proeutectoid ferrite.
Most investigations into the effect of heat input on low alloy steel weld deposits kgree that heat inputs near the top of the range used in the present work (1.2 - 4.3 KJ/mm) are detrimental to fracture toughness.29 ~32 This has been attributed to a smaller degree of grain refinement and a decrease in cooling rate accompanying increases in heat input. B E N N E T T 3 2 on the other hand, found that the impact properties of minimum dilution submerged arc welds in 19 mm mild steel plate prepared with consumables similar to those used for the CX series within the heat input range 1.9 - 14.2 KJ/mm were on optimum at 3.5 KJ/mm and deteriorated progressively above and below this value. The present results were in general agreement with this with the weld metals
produced at 2.2 and 4.3 KJ/mm having similar and better impact properties than the weld metal produced at 1.2 KJ/mm. The results for the BY series followed the trend of previous investigations with the 1.2 KJ/mm weld metal having slightly better properties than the 2.2 and 4.3 KJ/mm weld metals.
The impact properties of the BY series were improved by post-weld heat treatment at 650 C and thus substantiates the claims of the
manufacturer. This was accompanied, however, by a reduction in strength and hardness. These changes can be attributed to the precipitation and coarsening of carbide particles resulting in a lowering of the lattice friction stress. The slightly reduced impact performance after post-weld heat treatment at 550 C may be a result of secondary hardening by Mo„C. This effect would be removed at higher heat treatment temperatures as coarsening of the Mo„C particles occurs.
In contrast the impact properties of the CX series deteriorated after post-weld heat treatment at 550 C and 650 C and was accompanied by
small increases in strength and hardness. This is probably a secondary hardening effect due to the presence of V in the weld deposit. Similar behaviour has been reported for other low alloy steel weld deposits containing v22,32 ajjjj also in the HAZ of steels containing V.^»^^»-^^ Water quenching after post-weld heat treatment at 650 C resulted in inferior impact properties of the CX series compared with the equivalent air cooled condition. The slightly higher values of strength and hardness associated with the water quenching treatment suggests that this is due to a further increase in the lattice friction stress. The continued application of the preheat/interpass
temperature at 150 C for periods up to one month after welding is completed had no detrimental effect on the mechanical properties of the CX series.
lt would appear, therefore, that heat inputs above the range at present recommended for submerged arc welding of HY80 and Navy Ql steels
(1.2 - 2.2 KJ/mm) could be used to improve the economics of the welding operation without impairing the fracture toughness of the weld metal or HAZ. Some caution is necessary in applying this recoimnendation to QT35 steel
because of the impairment to fracture toughness of the HAZ at high heat inputs. It may be possible, however, to tolerate low fracture toughness in the HAZ because of its small size since the development of plastic zones at the tip of stress-raisers located in this region may spread into regions of
inherently better ductility.
The effect of post-weld heat treatment on fracture toughness and strength shows that such a treatment is not desirable for weld metals containing V and can probably not be justified economically on the V-free low alloy steel weld deposit. The best combination of strength and toughness will be achieved in the as deposited condition for both the BY and the CX series of weld metals.
It is suggested that the next stage in approving the recommendation for higher heat input for submerged arc welding of quenched and tempered steels should be to assess the weld metal properties using full thickness tests such as the crack opening displacement test, the Pellini drop weight test, or the
dynamic tear test since a number of instances of conflicting results between these tests and the Charpy test have been reported in the literature.3>13,32,37
The impact properties of the AX series of weld metals were not significantly affected by variations in heat input within the range 1.2 - 4.3 KJ/mm in agreement with previous work on mild steel weld deposits.3° However the strength decreased with increasing heat input indicating that heat inputs at the lower end of the range will give the best combination of toughness and strength. Post-weld heat treatment did not affect impact performance at any of the heat inputs used, but in the case of the 1.2 KJ/mm heat input it produced a decrease in strength while the opposite effect occurred with the 4.3 KJ/mm heat input. It would appear, therefore, that post-weld heat treatment of this weld metal is not necessary.
CONCLUSIONS
The BY series of weld metals had superior toughness and strength to the CX series over the heat input range
1.2 - 4.3 KJ/mm. This was related to flux basicity, weld metal composition, weld metal cleanliness and weld metal microstructure.
The BY series had marginally better toughness at 1.2 KJ/mm heat input than at 2.2 and 4.3 KJ/nm heat inputs.
The CX series had marginally better toughness at 2.2 and 4.3 KJ/mm heat inputs than at 1.2 KJ/mm heat input.
The strengths of the BY and the CX series of weld metals were not significantly affected by variations in heat input.
The toughness of the BY series improved after post-weld heat treatment at 650 C but this was accompanied by reductions in strength and hardness.
The toughness of the CX series deteriorated after post-weld heat treatments at 550 C and 650 C and was accompanied by small increases in strength and hardness. This was associated with the presence of vanadium in the weld
deposit. These changes were more marked when the post-weld heat treatment was followed by water quenching.
The mechanical properties of the CX series were not affected by the continued application of the preheat/interpass
temperature at 150 C for periods of up to one month after welding.
The toughness of the AX series of weld metals was not
significantly affected by either variations in heat input or by post-weld heat treatments. However strength generally decreased with increasing heat input and after post-weld heat treatment.
7. ACKNOWLEDGMENT
The authors wish to express their appreciation to the Ministry of Defence (Navy Department) for the support of this study, to Dr. R.L. Apps, Mr. I.M. Kilpatrick, and Mr. W.H. Winn for helpful discussions, and to Mr. D. Timpson and Mr. M. Wright for assistance with the experimental work. The authors would like to stress that
the opinions expressed in this report are their own and do not
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Charpy V-notch specimens
Tensile test specimens
2 0 0 - X X 0 o • • a s welded temDererl 1 hr at 55(PC tempered Ihr at 650°C -AO 0 AO 80 - 8 0 -AO 0 Temperature "C 200 -160 - 1 2 0 - 8 0 -AO -AO 0 AO 80
FIG. 3. CHARPY TRANSITION CURVES FOR BOSTRAND MS 65/ENCREX WELD DEPOSITS REGION OF MINIMUM DILUTION
X X 0 0 • • a s w e l d e d tempered I h r a t 550°C tempered I h r at 650°C 160
3 1201"
o c o o a CL)c
in 80 AC"/
^ / f 120 -AO0
AO
80
-80^ -AO„ 0
Temperature C 4 0 -AO 160 - 8 0 -AOFIG. 4. CHARPY TRANSITION CURVES BOSTRAND MS 65/ENCREX WELD DEPOSITS REGION OF MAXIMUM DILUTION
1 V W 0 O • • as welded 1 tempered 1 hr at 5 5 0 ° C tempered Ihr at
650°C 1
o / o =TÜO - ^ =fc- *u
kl-•
f /
-100 • / / -6Ó -2ÓJ
X
-1
A
-1
20iW
* -^0 20 160 12 0t
D O c o Q.803
O >^ xy> C - A O T e m p e r a t u r e . "C0 o • • a s welded tempered Ihr at 5 5 0 % tempered Ihr at 6 50°C 160 -120 80 AO -100 - 6 0 - 2 0 20 Temperature °C •100 - 6 0 - 2 0 2 0
FIG. 6. CHARPY TRANSITION CURVES FOR SD3/OP41TT WELD DEPOSITS - REGION OF MAXIMUM DILUTION
60
0 0 • • a s welded 1 tempered Ihr. at 550°C t e m p e r e d Ihr at 650^0 120 3 O a o£• 80
o « .o Cd >» 00 u V a cd AO - A OO
AO 8 0 - A O ^ O
Temperature,. °C y • AO 8 0 -AO O AO 80FIG. 7. CHARPY TRANSITION CURVES FOR BOSTRAND 31/ENCREX WELD DEPOSITS -REGION OF MINIMUM DILUTION
1 6 0 - X X o o 'o o as welded tempered Ihr. at 550°C tempered lhr.at650°C - 120 r-l 3 O - > a o a. u o « >» :« u a 8 0 AO 120 Temperature °C
FIG. 8. CHARPY TRANSITION CURVES FOR BOSTRAND 31/ENCREX WELD DEPOSITS -REGION OF MAXIMUM DILUTION
o
• t e m p e r e d I h r a t 650°C, air cooled
o tempered I h r a t 650°C, water quenchec 160
A O -120 80 -AO
±
±
±
- 6 0 -AO -200 20
Temperature "CAO
60
80
FIG. 9. EFFECT OF COOLING RATE AFTER TEMPERING AT 650°C ON CHARPY TRANSITION CURVES OF BOSTRAND 31/ENCREX WELDS DEPOSITED AT 2.2 KJ/mm - REGION OF MINIMUM DILUTION.
1 6 0
-4 • O
heat treated at 150°C for I d a y heat treated at 150°0 for 1 week
heat t r e a t e d atlSO^C for 1 month -160
5120 tempered 150°C / Imonth tempered 150°C / I w e e k - ? " ' ^ ' ' ^ ^^y^ a s w e l d e d tempered 1 5 0 % / I d a y .120 O cr 8 0 ^ o 3 O c
AO J
t^-'
Temperature °CFIG. 10. EFFECT OF PROLONGED HEAT TREATMENT AT 150°C FOR BOSTRAND 31/ENCREX WELD DEPOSITS REGION OF MINIMUM DILUTION.
.4
« j * «-lkltefei"'^:,^.:^v'-^>---...Jr^
(c) tempered 1hr at 550 C, cartMjn extraction replica, x 4500
Id) tempered 1 h at 650 C, carbon extraction replica, x4500
FIG. 11. Microstructures of Bostrand MS65/Encrex weld deposit Heat input 2.2 KJ/mm.
(a) as welded, optical x200 (b) as welded, carbon extraction replica x4500 ^
-X'^
.r^'j i / ' "^•jui •^i>i'^•r^
(c) tempered 1h at 5 5 0 0 , Carbon extraction replica, x4500
(d) tempered 1h at 6 5 0 C, carbon extraction replica, x4500.
FIG. 12. Microstructures in Oerlikon SD3/IN. '/^Mo/OPWITT weid deposit. Heat input 2.2 KJ/mm.
(c) tempered Ih at 550*'c, carbon extraction
replica, x4500 (d) tempered Ih at 650 C, carbon extraction
replica, x4500
FIG. 13. Microstructures of Bostrand 31/Encrex weld deposits. Heat input 2.1 KJ/mm.
