2 O JUN11968
CoA NOTE MAT. No. 18^o2i> HS DELFT
.^^i?.-'
THE C O L L E G E OF A E R O N A U T I C S
C R A N F I E L D
THE DESIGN AND CONSTRUCTION OF A WELD H E A T - A F F E C T E D ZONE SIMULATOR
by
CoA Note Mat. No, l 8 February_^_1968
THE COLLEGE OF AERONAUTICS DEPARTMENT OF MATERIALS
The d e s i g n and c o n s t r u c t i o n of a weld h e a t - a f f e c t e d zone s i m u l a t o r
b y
-T . E . C l i f t o n and M . J . George
S_U_M_M A R Y
I n v e s t i g a t i o n of t h e s t r u c t u r e and p r o p e r t i e s of the h e a t - a f f e c t e d zones i n welded j o i n t s i s u s u a l l y l i m i t e d by t h e i r s m a l l s i z e and t h e i r c o m p l e x i t y . One method of overcoming t h i s problem i s to s i m u l a t e t h e s t r u c t u r e a t a
p a r t i c u l a r p o i n t i n t h e h e a t - a f f e c t e d zone i n a specimen of l a r g e r s i z e by imposing on i t t h e t h e r m a l c y c l e s u s t a i n e d a t t h a t p o i n t .
The eqiiipment d e s c r i b e d i n t h i s n o t e u s e s a . c . r e s i s t a n c e h e a t i n g and w a t e r c o o l i n g to impose t h e r m a l c y c l e s on 2-5" x O.V x O.k" s p e c i m e n s , the
t h e r m a l c y c l e b e i n g chosen by a d j u s t m e n t of a bank of v a r i a b l e r e s i s t o r s t o c o n s t r u c t a v o l t a g e a n a l o g u e . C o n t r o l of specimen t e m p e r a t u r e i s a c h i e v e d u s i n g a t h y r i s t o r and two i g n i t r o n s t o c o n t i o l the i n p u t a t ^^i-OV. t o a
welding t r a n s f o r m e r . Feedback i s a p p l i e d from a thermocouple welded t o the specimen h o t - z o n e . The e q u i p n e n t h a s been shown t o produce the d e s i r e d t h e r m a l c y c l e s i n a r e p r o d u c i b l e manner.
Summary
Introduction
The general principles of the design
The specimen grips
The control circuit
Assessment of simulator performance
Further applications of the simulator
Appendix
Acknowledgement s R e f e r e n c e s
1
-Introduction
As welding techniques become more and more refined, and welded s t r u c t u r e s are put i n t o service in applications which demand increasing degrees of r e l i a b i l i t y e . g . n a t u r a l gas p i p e l i n e s , nuclear engineering, i t i s important t h a t a l l p o s s i b l e techniques should be used t o increase our understanding of the factors underlying s a t i s f a c t o r y welded-joint performance.
A welded j o i n t contains zones which have had widely differing
thermal h i s t o r i e s , and which may show, t h e r e f o r e , very d i f f e r e n t mechanical p r o p e r t i e s . Three major zones e x i s t :
-i ) The weld metal, wh-ich has been -in the molten s t a t e , and wh-ich shows the general c h a r a c t e r i s t i c s of cast m a t e r i a l ;
i i ) The parent m a t e r i a l , with microstructure and p r o p e r t i e s t o t a l l y unaffected by the heating sustained during welding;
i i i ) The h e a t - a f f e c t e d zone, which l i e s between the fusion boixndary on one side and the unaffected parent metal on the o t h e r . Figure 1 shows these zones in a section taken through a butt-weld.
The heat-affected zone m a t e r i a l generally forms a very small p a r t of a welded structure as a whole, but may decide the o v e r a l l perfoimance of the
s t r u c t u r e , on a 'weal:est-link' b a s i s . Many welded s t r u c t u r e s have f a i l e d , u s u a l l y with expensive r e s i o l t s , because of the effect of the welding process on the immediately adjacent m a t e r i a l i . e . the heat-affected zone, which led t o a l o c a l reduction i n d u c t i l i t y and f r a c t u r e toughness, and allowed easy propagation of f r a c t u r e s o r i g i n a t i n g a t some welding or m a t e r i a l d e f e c t . For t h i s reason, the i n t e g r i t y of an e n t i r e welded s t r u c t u r e may depend on
the production, in the HAZ, of s a t i s f a c t o r y mechanical p r o p e r t i e s by c o n t r o l l i n g the m i c r o s t r u c t u r e . In order t o do t h i s , however, one must know:
-a) the s t r u c t u r e and p r o p e r t i e s of the HAZ in a p a r t i c u l a r m a t e r i a l , and b) which structuTes w i l l be unacceptable, and how they may be modified, far
insteince, by weld pre-heat or post-heat treatment
Much of the i n v e s t i g a t i o n a l work on the common s t r u c t u r a l s t e e l s i s of an ad-hoc n a t u r e , designed e i t h e r to e s t a b l i s h a s a t i s f a c t o r y welding procedure or to solve some p a r t i c \ i l a r f a b r i c a t i o n problan, and there i s a need for longer-term i n v e s t i g a t i o n s i n t o t h e behaviour of even the best-known s t r u c t u r a l s t e e l s , to gain an understanding of the h e a t - a f f e c t e d zones.
However the h e a t a f f e c t e d zones axe usually only of the order of m i l l i -metres in width, and are completely inhanogeneous i n n a t u r e , since the thermal cycles sustained change continually across the width of the heat-affected zone. For these reasons, i t i s not l e a s i b l e to cut t e s t - p i e c e s for the usual
mechanical t e s t s d i r e c t l y from a HAZ, although hardness surveys are
frequently c a r r i e d o u t , and notch toughness specimens have been cut t r a n s -versely from welded j o i n t s . Notches placed a t increasing distances from the weld fusion boundary give information on changes in impact toughness across the HAZ (l). However, these techniques a r e of limii:ed v a l u e .
A l t e r n a t i v e l y , we can aim to produce, in specimens l a r g e enough to allow conventional mechanical t e s t i n g , equivalent m i c r o s t r u c t u r e s , and hence mechanical p r o p e r t i e s , to those obtaining a t specific p o s i t i o n s
inside the HAZ, by Imposing the same thermal cycles sustained during welding. This note describes the design and construction of equipment to c a r r y out
such simulation.
Figure 2 shows the thermal cycles sustained a t various distances from the fusion boundary in a weld of 77>000 j / i n . heat input in -^f^' thick p l a t e , with no p r e - h e a t . The cycles were obtained by means of thermocouples
flash-welded i n t o p o s i t i o n p r i o r t o welding. The main feature i s a rapid r i s e to a peak temperature, which i s a function of distance from the fusion boundary, followed by cooling t o room temperature at an exponentially declining r a t e . In considering the v i s i b l e HAZ in s t e e l s , as shown i n Figure 1, we are concerned with peak tanperatures in t h e approximate range 7OO°-1500°C.
However, there appears t o e x i s t , outtide the v i s i b l e HAZ, a region of low d u c t i l i t y in which thermal s t r e s s i n g beyond the y i e l d p o i n t , at temperatures of around 100°-500''C, has produced s t r a i n ageing ( 2 ) , and which should
properly be included within the term' heat-affected zone' .
The peak temperature achieved, and the r a t e s of heating and cooling,
vary widely in welding as a whole, and are controlled by such process v a r i a b l e s as the type of welding process used, the heat input per inch of weld, the
p l a t e thickness and the degree of pre-heat used. In a d d i t i o n , there i s a r a p i d drop in peak temperature and heating and cooling r a t e s , as the distance from the fusion boundary increases (Figure 5 )
-Tne p r o p e r t i e s of the HAZ produced by a weld run a r e affected by the heating from subsequent r u n s , in a multi-run weld, and by any post-weld h e a t - t r e a t m e n t . In g e n e r a l , the effects of both of these are b e n e f i c i a l . The general p r i n c i p l e s of the design
In order to impose thermal cycles of the type shown in Figure 2, the prime requirement was the supply and a b s t r a c t i o n of large amounts of heat t o the specimen, i n a short time and in a c o n t r o l l e d fashion,
Tv/o p o s s i b l e methods of heating were high frequency induction heating and r e s i s t a n c e h e a t i n g . Induction heating at the necessary r a t e s would be d i f f i c u l t , and the well-kno^m h / f skin-heating effect could produce inhomogeneous temperature d i s t r i b u t i o n s w i t h i n the specimen. In a d d i t i o n , the c a p i t a l cost of heating equipment would be r e l a t i v e l y high. Neverthe-l e s s , h / f induction h e a t i n g was used for at Neverthe-l e a s t one HAZ simuNeverthe-lator ( 5 ) .
5
-D.C. r e s i s t a n c e heating would be i d e a l in providing uniform heating a t the necessary r a t e s but the provision of the necessary power at low
impedance i s a considerable problem. A.C. r e s i s t a n c e h e a t i n g , using a welding transformer as an impedance converter, i s a ready source of the necessary power, although i t has the drawbacks of skin-heating e f f e c t s , the s e v e r i t y of which v a r i e s with temperature and heating r a t e s , and the p o s s i b i l i t y of i n t e r f e r e n c e , produced by the l a r g e a . c . f i e l d s , in the operation of e x t e r n a l c i r c u i t s , n e c e s s i t a t i n g shielding p r e c a u t i o n s .
However, low frequency a . c . heating has been used with success in developing other HAZ simulators (6,7) and t h i s technique was chosen for the present work.
Cooling was achieved by c i r c u l a t i n g water, a t f u l l mains pressiire, through copper specimen g r i p s , ensuilng t h a t the n a t u r a l cooling r a t e , i . e . in the absence of any h e a t i n g , was i n excess of those necessary f x simiilation of the required thermal cycles, so that control of the cooling r a t e could be maintained by a v a r i a b l e amount of heat input to t h e specimen during c o o l i n g .
The general design requirements w e r e :
-i ) a heat -input v a r -i a b l e over a w-ide range dur-ing the cycle; i i ) accurate control of heating and cooling r a t e s ;
i i i ) maximum r e p r o d u c i b i l i t y between successive runs a t a given s e t t i n g of the c o n t r o l s ;
iv) easy v a r i a t i o n of the demand c y c l e , with a minimum of readjustment; v) a record of the tsnperature cycle imposed on each specimen, as a check
on accuracy and r e p r o d u c i b i l i t y of the thermal c y c l e .
The specimen i s O.k" square by 2.5" long, l a r g e enough to allow a Charpy impact specimen or a small t e n s i l e specimen t o be machined from i t , a f t e r h e a t -treatment. The h e a t - t r e a t e d voliome i s approximately a O.V cube in the
specimen c e n t r e . Heating and cooling r a t e s were more l i k e l y t o be unifoim in a cube-shaped element.
3 . The specimen g r i p s
The specimen g r i p s c o n s i s t of four 2" x 2" x IT'/Q copper blocks, through which water i s c i r c u l a t e d at f u l l mains pressure . Each end of the specimen
i s gripped between two blocks by means of four 5/16" b o l t s (Figure k). In e a r l y t r i a l s , i t was found üiat the r e s i s t a n c e across the specimen grips
affected the peak temperature achieved a t a given control s e t t i n g , giving poor specimen-to-specimen repix)ducibility. To standardize r e s i s t a n c e as far as p o s s i b l e , soft aluminium packing pieces are used on each face of the specimen
in contact with a copper block, and the b o l t s a r e tightened t o 50 l b s - i n s . by means of a torque wrencn.
The copper blocks are moionted on a 1^/e" thick Tufnol block, backed with ^/e" s t e e l p l a t e to r e s t r a i n the thermal expansion of t h e specimen dialing c y c l i n g .
This approximates, to some e x t e n t , at l e a s t , to the r e s t r a i n t on the HAZ i n an a c t u a l weld j o i n t .
Connections to the transformer were made as short and thick as
p o s s i b l e (l-g-" x l" x 5" long) to minimise the voltage drop. The voltage across the transformer output terminals i s kY. approx. and across the specimen grips 1.5 V. approx. Current taken during t h e heating portion of the cycle may be as higli as 15,000 A.
A modification t o t h i s arrangement was made during work on the effect of v a r i a t i o n i n r e s t r a i n t (U), in which one end of the specimen was allowed to move during expansion and contraction. One p a i r of blocks was mounted on a b a l l - b e a r i n g s l i d e and the power fed in via f l e x i b l e connections. k. The control c i r c u i t
B a s i c a l l y , the control c i r c u i t continually compares the specimen temperature, measured by a thermocouple, with the required thermal cycle, as given by a function generator. The p o l a r i s e d e r r o r s i g n a l thus obtained
i s used to control the heat input to t h e specimen.
This c o n t r o l c i r c u i t nay be considered in two s e c t i o n s : -a) The function generator and conparison network
I t was necessary to generate a voltage analogue of the required temperature c y c l e , which was then compared continuously with the e.m.f. produced by the thermocouple attached t o the specimen, i . e . with the specimen temperature. As the development of the simulator progressed, t h i s was achieved in three
d i f f e r e n t ways.
O r i g i n a l l y , a cam, obtained by p l o t t i n g the chosen experimental time-teniperature curve i n a polar fashion, was r o t a t e d through 560°C, at the correct constant speed. By connecting a cam follower to the wiper of a 10 KÜ l i n e a r potentiometer and using a s u i t a b l e voltage supply across the potentiometer, a voltage-time r e l a t i o n s h i p matching the desired temperature-time curve was produced. Owing to mechanical problems, such as backlash, and the labour of constructing t h e l a r g e number of cams required, t h i s method was eventually superseded. Correction of e r r o r s occurring in cycles with
rapid heating r a t e s (discussed l a t e r ) required on-the-spot ' t r i a l and e r r o r ' modifications of the ceim shape, and t h i s d i f f i c u l t y contributed to the decision
t o discard t h i s method. However, the cam u n i t served s a t i s f a c t o r i l y as the function generator during one programme of research (k).
I t was replaced by a simpler function generator, i n which a s i n g l e - t u r n potentiometer with Jik t a p s , equally spaced over i t s r e s i s t a n c e range,was driven a t a constant speed over i t s 5to°C a r c . Tl^e potentiometer (A) i s shown
with the driving motor ( B ) , in Figure 5- A v a r i a b l e r e s i s t o r was connected i n s e r i e s with the potentiometer and a s t a b i l i s e d d . c . p o t e n t i a l connected as shown in Figure 8, By adjusting the tap to which the p o s i t i v e voltage
5
-connection to the network i s made, and by adjusting the value of the s e r i e s r e s i s t o r , the shape of the output function can be varied with respect to the proportion of the cycle time occupied in r i s i n g t o the pealc, and the
magnitude of the f i n a l output v o l t a g e , r e s p e c t i v e l y . F u r t h e r , by modifying the t o t a l r e s i s t a n c e value at each tap p o s i t i o n , by s m t a b l e p a r a l l e l fixed-value r e s i s t o r s , the voltage-time function can be modified to produce a shape equivalent to that of the required experimental curve. Although t h i s i s constructed from a s e r i e s of s t r a i g h t l i n e s between adjacent tap p o s i t i o n s , a very s a t i s f a c t o r y approximation i s obtained. The method used to c a l c u l a t e the values of the p a r a l l e l r e s i s t o r s i s ^own in the Appendix.
The major drawback was in the s e l e c t i o n of s u i t a b l e fixed value r e s i s t o r s , together with the labour of constructing a r e s i s t o r bank for each demand
cycle r e q u i r e d . Adjustment of e r r o r s produced by high heating and cooling r a t e s , as b e f o r e , was also d i f f i c u l t . Accordingly, the equipment i n i t s l a t e s t form, on the r i g h t of Figure 6, contains a bank of m u l t i - t u r n potentiometers connected in p a r a l l e l wiüi the segments of the function generator potentiometer. During the l a t t e r p a r t of the c y c l e , the slope of the required fimction i s generally small, requiring p a r a l l e l r e s i s t o r s of low value. For t h i s reason, several of the p a r a l l e l potentiometers span two segments towards the end of the cycle . A diagram showing the lay-out of the p a r a l l e l potentiometers, giving t h e i r values and span i s given in the Appendix. Sockets have been provided t o enable fixed value r e s i s t o r s to be connected in s e r i e s with the f i r s t six h e l i c a l potentiometers, when the calculated r e s i s t a n c e i s outside the range of the potentiometer.
Taps 2 to 7 ^^^ connected t o the supply p o s i t i v e through a s i n g l e pole six-way switch (C in Figure 5) which allows the peak p o s i t i o n to be s e l e c t e d . In a d d i t i o n , the r o t a t i o n a l speed of the function generator can be adjusted, allowing the s e t t i n g - u p of a wide v a r i e t y of cycles on the s i n g l e apparatus.
The voltage analogous t o the specimen ten^ierature, i . e . the feedback s i g n a l required to close the l o o p , i s obtained by amplifying the voltage output from a fine Chromel-Alimie] thermocouple, capacitor-discharge welded on to the centre of the heated zone of t h e specimen.
The thermocouple output i s connected i n t o a single channel Graphispot r e c o r d e r , shown on the l e f t in Figure 6 , which, as well as producing a record of the temperature cycle sustained by the specimen, gives an output voltage proportional t o the pen displacement. This i s achieved by using a 10 IÖÏ
l i n e a r potentiometer mechanically connected across the scale of the instrument, with t h e wiper connected t o the pen c a r r i a g e . The voltage across the
potentiometer i s provided e x t e r n a l l y , and can be varied a s r e q u i r e d , providing a high-gain d . c . amplifier for the thermocouple e.m.f.
The supply voltages across the function generator and Graphispot potentiometers are chosen to give e r r o r voltages of the correct magnitude t o feed d i r e c t l y i n t o the heat control u n i t , and thus are a means of c o n t r o l l i n g the s e n s i t i v i t y of the h e a t i n g c i r c u i t to d e v i a t i o n s from the required temperature.
b) The h e a t - c o n t r o l unit
The h e a t - c o n t r o l unit was designed to accept signal voltages betvreen zero v o l t e (maximum power) and +10 v o l t s (minimum power). As the error s i g n a l approaches zero, the demand w i l l be f o r decreasing power, therefore a datum s h i f t , provided by a s u i t a b l y - p o l a r i s e d fixed-value d . c . v o l t a g e , i s r e q u i r e d .
The u n i t was o r i g i n a l l y supplied as a modification of a commercial s p o t -welder control u n i t , but a f t e r u n s a t i s f a c t o r y performance and frequent
component f a i l u r e , i t was redesigned by the Instrumentation Section of ElectricEil Department, for whose help we are most g r a t e f u l . The u n i t is now performing s a t i s f a c t o r i l y .
As the method used in c o n t r o l l i n g the power to the primary of the welding transformer i s r e l e v a n t t o many control problems encountered in welding
techniques, the type of c i r c u i t employed w i l l be described more f u l l y . A c i r c u i t diagram showing d e t a i l s of the heat control unit i s shown in Figure
9-A 12 volt r . m . s . voltage in phase with the i g n i t i o n supply is full-wave r e c t i f i e d to g i v e , v i a a pulse forming network, comprising t r a n s i s t o r Tx and a s s o c i a t e d components, a pulse of 100 c . p . s . on the base of t r a n s i s t o r T3. T r a n s i s t o r T2 i s a constant current generator charging the O.kJ iiF capacitor
to give a voltage remip dependent upon the time constant of öie network. The current i s turned on with the i n i t i a t i o n contacts closed. Normally, Tx is turned on, cutting off T3, so t h a t a t tiie beginning of each h a l f - c y c l e , the action of Tx and T3 i s to discharge the c a p a c i t o r . Therefore, a ramp i s produced which i s r e s e t at the beginning of each h a l f - c y c l e . This ramp i s applied, v i a an e m i t t e r follower t o a Schmitt t r i g g e r network whose hold off b i a s i s a function of the control s i g n a l .
By t h i s means the t r i g g e r i s made t o operate at a time a f t e r the beginning of each ^ cycle depending upon the value of the control v o l t a g e . The Schmitt t r i g g e r p\ilse f i r e s a blocking o s c i l l a t o r , which produces the necessary f i r i n g pulse for the t h y r i s t o r .
kkO volts i s connected, via the routing diodes Dx, D2, D3 and D4, t o the appropriate i g n i t r o n i g n i t o r , through the s i n g l e t h y r i s t o r , the magnitude of the r . m . s . current to the transformer primary being determined by the time delay, as set by the d . c . control voltage ( e r r o r s i g n a l ) . The function of the Zener diode, in s e r i e s with the t h y r i s t o r , i s t o ensure t h a t the t h y r i s t o r current goes to z e r o , and thus e x t i n g u i s h e s , a t the end of each h a l f - c y c l e . This is necessary because of the inductive nature of the load, which produces phase s h i f t between load current and v o l t a g e , and would normally prevent e x t i n c t i o n a f t e r each h a l f - c y c l e . The c a p a c i t o r - r e s i s t o r combination in p a r a l l e l with the t h y r i s t o r i s designed as a t r a n s i e n t f i l t e r , t o prevent
spurious voltage spikes from f i r i n g the device.
The power supply to t h e mains transformer i s therefore controlled by two I g n i t r o n s , connected back-to-back, so t h a t each f i r e s on successive h a l f - c y c l e s
_ 7
-of the supply voltage. The action -of the h e a t - c o n t r o l u n i t i s t o vary the proportion of each h a l f - c y c l e over which the i g n i t r o n s w i l l f i r e , as the s i g n a l voltage v a r i e s i n the range 0 to lOV.
The temperature control of the specimen i s therefore b a s i c a l l y proportional i n c h a r a c t e r , since the amount of power applied i s a d i r e c t function of the deviation of the specimen temperature from the instantaneous demand v a l u e . Because of the complex feedback loop, there i s a tendency t o introduce a
time-lag i n t o the response of the system. Further, since the desired temperature i s constantly changing the effect of a time-lag i s to d i s t o r t the thermal cycle imposed on the specimen. Since the l a g i s g r e a t e s t at high heating r a t e s , the effects are seen most in the peak temperature region. The parameters of the cycle most affected are the r a t e of h e a t i n g , which w i l l be higher than that required, and the peak temperature, which w i l l be exceeded-This effect i s p a r t i a l l y overcome by a l t e r i n g the voltage across the Graphispot
and function-generator potentiometers u n t i l the peak temperat\Are is achieved. Since the time-lag v a r i e s throughout the cycle with change i n heating and
cooling r a t e s , the voltage adjustment which gives the correct peak temperature gives an e r r o r elsewhere, notably a droop in the curve a t lower temperatures towards the end of the cycle. When cams were used, t h i s was corrected by modifying the appropriate section by f i l i n g and in t h e l a t e r v e r s i o n s , using the s e r i e s variable r e s i s t o r to give dhe correct voltage at the end of the c y c l e . This can be simplified by adding a d e r i v a t i v e control term to t h e basic
p r o p o r t i o n a l term, t o give an autonatic c o r r e c t i o n of the s i g n a l voltage, with a maximum effect at high heating r a t e s and a minimum at the lower r a t e s towards the end of cycle. I t was fo\md, however, t h a t the magnitude of the d e r i v a t i v e component required to achieve control of the i n i t i a l heating r a t e was so l a r g e as t o introduce i n s t a b i l i t y i n the degree of control elsewhere in the cycle, p a r t i c u l a r l y very near to the peak p o s i t i o n , where the d e r i v a t i v e canponent
i s removed very r a p i d l y , producing an o v e r - r e a c t i o n of the control system and a double-pealc effect on the specimen thermal cycle.
Ad-hoc modification of the shape of the thermal c y c l e , by alteririg the p a r a l l e l r e s i s t o r v a l u e s , achieves the same r e s u l t in a r e l a t i v e l y short time, and t h i s method of adjusting the shape of the thermal cycle proved a viable p r o p o s i t i o n , in p r a c t i c e .
5. Assessment of simulator performance i ) Heat control u n i t
The modified heat control u n i t operated very s a t i s f a c t o r i l y during the t e s t period. Adjustment of the heating r a t e during the cycle occurred smoothly and there \iBie a general lack of any random f i r i n g by mains t r a n s i e n t s which was
a too-frequent occurrence with the o r i g i n a l u n i t . i i ) Accuracy of Control
This was d i f f i c u l t to a s s e s s q u a n t i t a t i v e l y during a continuously changing demand cycle, b u t on a q u a l i t a t i v e b a s i s , the degree of control appeared to be
s u f f i c i e n t , where a true control s i t u a t i o n applied, e . g . if the p o t e n t i o -meter drive motor was h a l t e d during the cooling p e r i o d , the specimen was held a t a constant temperature, with no noticeable f l u c t u a t i o n on the Graphispot record. The response of the control c i r c u i t is adjustable by v a r i a t i o n of the voltages across the function generator and Graphispot potentiometers, and an optimum range of values l i e s j u s t below voltages a t which o v e r - r e a c t i o n occurs, giving 'hunting' of the temperature.
I f true control could be achieved a t a l l points on the thermal cycle, as would be the case with slow heating r a t e s , the voltages across the function generator and Graphispot potentiometers would be approximately equal. With the heating r a t e s encountered i n the cycles sho-^m i n Figure 2, up to around 600°C/Eecond, a small time-lag b u i l t up in the peak temperature region, giving a peak temperature higher than the required v a l u e . Correction was applied by making the voltage across the function generator lower than t h a t across the Graphispot potentiometer. This then introduced d i s t o r t i o n in the l a t t e r part of the curve, where t n i e control was achieved.
i i i ) Reproducibility
Successive demand cycles were found to be e s s e n t i a l l y i d e n t i c a l .
Specimen-to-specimen f l u c t u a t i o n s of the thermal cycle produced did, however, occur, although these were f e l t t o be due more t o such factors as inconsisten-cies i n specimen set-up or i n mains v o l t a g e , r a t h e r than t o deficiency of any p a r t of the control c i r c u i t .
Heating, p a r t i c u l a r l y in the early s t a g e s , was observed t o occur mainly due to i n t e r f a c i a l heating,which could obviously vary from one specimen to the n e x t , affecting the efficiency of heating during the c r i t i c a l and l e a s t -c o n t r o l l e d se-conds leading up to the peak value. Aluminium pa-cking pie-ces were necessary to give even e l e c t r i c a l contact between the specimen and the water cooled blocks, i n order to prevent sparking during periods of high h e a t i n g r a t e s . To s t a n d a r d i s e , as f a r as p o s s i b l e , i n t e r - f a c i a l c o n d i t i o n s ,
fresh packing pieces were used for each specimen, and ihe clamping nuts tightened evenly and to a constant 50 l b . i n . torque, using a torque wrench.
Applying these p r e c a u t i o n s , i t was possible to produce sets of specimens with e s s e n t i a l l y i d e n t i c a l thermal cycles. For i n s t a n c e , 24 specimens were produced having had the 940°C peak temperature cycle, with pealc temperatures lying within ± 25°C of 9l<-0°C. Twelve specimens in which the peak temperatures a l l lay within ± 10°C of t h a t required, i . e . within an error of ± 1$ were
s e l e c t e d f o r Charpy impact t e s t i n g .
Because of the l a r g e s c a t t e r in the Charpy t e s t , i t was most important t h a t , as near as was p o s s i b l e , the specimens were i d e n t i c a l . The specimens with l e s s accurate cycles were kept for l e s s demanding u s e s , such as t e n s i l e
t e s t i n g and metallography.
iv) Variation of demand cycle
9
-analogue of the desired cycle can be calculated, using the method given
i n the Appendix. Since velocity feedback proved impractible, the d i s t o r t i o n produced with f a s t heating r a t e s was corrected by t r i a l - a n d - e r r o r modification of the p a r a l l e l r e s i s t o r s in the d i s t o r t e d region. This modification was akin to the modification of cam shapes which was necessary when the driven-cam function generator was in use {k), but was much e a s i e r to carry out.
Depending on the degree and extent of d i s t o r t i o n p r e s e n t , 1 - 2 hours was s u f f i c i e n t to give a s a t i s f a c t o r y f i t to the experimental curve. A
subsequent specimen production r a t e of about 10 minutes per specimen enables a set of 2k specimens to be produced in 1 - 1-|- worMng days, thus providing the raw m a t e r i a l for a considerably longer period of further work. Specimen-production time i s , t h e r e f o r e , a r e l a t i v e l y small f r a c t i o n of the t o t a l time of a research programme. Figures 10, 11 and 12 show the three uppermost curves of Figure 2 , together with the simulated thermal cycles.
6. Further a p p l i c a t i o n s of the simulator
Because of the high heating and cooling r a t e s a v a i l a b l e , the close temperature c o n t r o l , and the a b i l i t y t o programme a heat treatment c y c l e , the simulator can be used q u i t e generally as a heat-treatment f a c i l i t y .
In p a r t i c u l a r heat-treatments r e q u i r i n g continuous or discontinuous v a r i a t i o n of temperature are e a s i l y and quickl--' c a r r i e d out, making i t a very useful research t o o l . . During long heating p e r i o d s , decarburisation and oxidation are l i k e l y to occur. The enclosure of the specimen g r i p s i n a vacuum chamber, allowing treatment in vacuo or under c o n t r o l l e d atmosphere, i s a possible
f u r t h e r development.
With f=uitable modification of the specimen g r i p s , other techniques which might be employed a r e , a) high speed dilatometry, t o a s c e r t a i n the true t r a n s -formation temperatures applying to the heating and cooling r a t e s encountered i n HAZ thermal c y c l e s , and b) t e n s i l e t e s t i n g a t high temperatures.
Appendix
Calculation of r e s i s t a n c e analogue
Figure Al represents the function generator (tapped potentiometer) without the p a r a l l e l r e s i s t o r s .
With Ex = E2 With wiper B at A
E3 = 0
With wiper B at T2 and wiper X at Ta E3 = Ex = E2
With wiper B at C and wiper X a t T2
E3 = Ex - E2 (Ta-C/rp J)) (T2C represents t o t a l r e s i s t a n c e ^ between point T2 and C).
Figure A2 shows a s e r i e s of curves representing output voltage E3 p l o t t e d against wiper B p o s i t i o n , for d i f f e r e n t connections of wiper X. A similar graph may be constructed with r e s i s t a n c e between A and B, figure Al, replacing voltage on the Y a x i s . Then by modifying the r e s i s t a n c e value at tap
p o s i t i o n s , throughout the whole range of the potentiometer, a curve of the desired shape can be constructed. The curve w i l l be constructed from a
s e r i e s of s t r a i g h t l i n e s between adjacent tap p o s i t i o n s , as shovrn i n Figure A5. The p o s i t i o n chosen for wiper X (Figure Al) w i l l depend upon the shape of the thermal cycle r e q u i r e d .
As a general r u l e , for slow heating r a t e s a high tap p o s i t i o n should be s e l e c t e d , and for high heating r a t e s , a low tap p o s i t i o n i s more advantageous.
Figure 3 shows t h a t for slov/ heating r a t e s , e s p e c i a l l y when the heating curve deviates markedly from a s t r a i g h t l i n e , a g r e a t e r number of segaents are needed t o simulate closely the desired function.
The speed of r o t a t i o n of the function generator should be selected to s a t i s f y the following conditions:
(a) The peak of the r e s i s t a n c e ciurve should f a l l as nearly as possible to a tap p o s i t i o n , ( e s p e c i a l l y important for higli heating r a t e s ) ,
(b) The time for a complete revolution should correspond to the cycle time for the thermal cycle r e q u i r e d .
Having selected a p o s i t i o n of wiper X and the cycle time of the function g e n e r a t o r , the experimental temperature cycle should be p l o t t e d t o t h e above
11
-tirüe b a s e , marking on t h e X a x i s t h e p o s i t i o n s of t h e p o t e n t i o m e t e r s t a p s . The Y a x i s i s t h e n c o n v e r t e d from thermocouple o u t p u t t o r e s i s t a n c e , u s i n g a peak value of between 7OO and 80Cn, and e v a l u a t e the r e s i s t a n c e a t t a p p o s i t i o n s .
The r e s i s t a n c e of t h e f u n c t i o n g e n e r a t o r p o t e n t i c m e t e r i s 2ÖKfl which g i v e s a r e s i s t a n c e of 590f^ between a d j a c e n t t a p s . For t h e m a j o r i t y of e x p e r i m e n t a l curves a peak v a l u e of 700 - 80Cfi g i v e s p a r a l l e l r e s i s t a n c e s
which a r e r e a l , and f a l l w i t h i n t h e range of t h e h e l i c a l p o t e n t i o m e t e r s f i t t e d . F i g u r e A^l- i s a t y p i c a l t e m p e r a t u r e time c u r v e , and t h e p r o c e d u r e f o r
c a l c u l a t i o n of p a r a l l e l r e s i s t a n c e v a l u e s i s d e t a i l e d below.
The p o s i t i o n of the X w i p e r i n t h e example was chosen f o r t a p 3^ w i t h the peak t e m p e r a t u r e c o n v e r t e d t o 800fi.
The r e s i s t a n c e a t t a p 1 must t o t a l l+lOfi.
i . e . 590fi i n p a r a l l e l w i t h r e s i s t o r Rx, must equal klCQ 590Ri
= ij-10 Rx- 134Cf2 590fRx
The r e s i s t a n c e a t t a p 2 must t o t a l 70Cfi
i.e.-59Cf2 i n p a r a l l e l w i t h r e s i s t o r R2 i n s e r i e s w i t h 4l0fi must e q u a l 7OOD
^1° ^ Ü f c = ^ ° ° ^2 - 570n
S i m i l a r l y t h e r e s i s t a n c e a t t a p 3 must t o t a l 80Cfi
7^°
+
5 9 ^ ^
="
ö°°
^^ - ^ ^
C o n t i n u i n g from the peak t a p p o s i t i o n t h e p r o c e d u r e i s somewhat m o d i f i e d . The peak i s c a l l e d z e r o , t h e curve i s i n v e r t e d ( V o l t a g e E2 opposes Ex) and t h e p a r a l l e l r e s i s t a n c e c a l c u l a t e d a s b e f o r e .
Figure A5 shows the i n v e r t e d curve of t h e example shown i n Figvire Ak The r e s i s t a n c e a t t a p k must t o t a l 6Cf2 ( i . e . 800-7*4-0)
| g j - = 60 R . - 66.SO
S i m i l a r l y t h e r e s i s t a n c e a t t a p 5 must t o t a l 2200 (8OO-580)
The value of the s e r i e s r e s i s t o r (CD in f i g . A.1) i s calculated as follows.
Total value of r e s i s t a n c e from peak p o s i t i o n to f i n i s h must equal the value from peak to s t a r t , which i s , in the example considered, 800fi.
Therefore: Value of s e r i e s r e s i s t o r = 800 - value a t f i n a l t a p . The t a b l e shows the value of each p a r a l l e l h e l i c a l potentiometer and
the relevant tap p o s i t i o n . Each potentiometer requires 10 complete revolutions to scan the t o t a l range and the turns i n d i c a t o r f i t t e d shows the precise
p o s i t i o n of the wiper.
Figure A6 shows the potentiometer layout on t h e front panel.
Towards the end of the cooling curve, where the slope i s small, p a r a l l e l r e s i s t o r s have been connected t o span, in some c a s e s , two segments.
This must be taken i n t o consideration i n the c a l c u l a t i o n s . The fixed value w i l l , i n these cases, be ll80fi instead of 590^ ,
Acknowledgements
The authors wish t o thank E. S i l l s and B. Moffat of the Instrimientation Section for advice and p r a c t i c a l a s s i s t a n c e i n the redesign of the heat control u n i t , and T. Biston of the Department of M a t e r i a l s , for carrying out the construction of t h e simulator.
References
1. A.C. de Koning I . I . W . Document IX-i)-71-65
2 . C.F. Tipper B r i t . Weld. J . , 1966, 13, P- k6l-k66, 3 . M.J. George Unpublished work, Cranfield, I966. k. M.D. Coward M.Sc. T h e s i s , London University, I967. 5. M. Inagaki et a l . Trans. Nat. Res. I n s t , Met. (Japan),
196k, 6, p . 39.
6. E.F. Nippes and W.F. Savage Weld. J., 19'49, 28, p. 534s.
7. E. Fletcher and D. Shaw Swinden Laboratories, United Steel Go's. Ltd., 1961. (Private Communication).
F!G. 1. MACROGRAPH OF BUTT WELD SHOWING POSITION OF HEAT AFFECTED ZONES ( x 2 MAGNIFICATION).
1200 r IBO XID 90O 700 500 I X • 300 K»L Distance Fusion 1 - T / j , 2-Vc2 3-T/c3 t-Vc5 5-T/r,6 6 - f / c 8 from OOS" 0 0 7 ' o-ir 0-13-O K ' 0 2 9 ' 15 20 25 10 35 1.0 i5
FIGURE 2 . EXPERIMENTALLY DETERMINED THERMAL CYaES.
50 55 Tim« (Sees)
o 1250 a. E 6» 1000 E 750 X o 500 \
V
1 ^.
<k
\ • V • ^ v ^ ^****«'«^^^ — ~ — ~» 250 O 3 6 9 12 15 18 Distance from Weld Fusion Boundary, mms.FIG. 3 RELATIONSHIP BETWEEN PEAK TEMPERATURE AND
THE DISTANCE FROM THE FUSION BOUNDARY. REFERENCE 4
FIG. 4 SPECIMEN GRIP DESIGN AND THE POSITION OF THE FEEDBACK THERMOCOUPLE
A - MULTI - TAP POTENTIOMETER. B - SYNCHRONOUS MOTOR.
C - PEAK - POSITION SELECTOR SWITCH.
FIG 5. - DETAILS OF THE FUNCTION GENERATOR.
DC. ERROR MOTOR THYRISTOR CIRCUIT I ; I TRANSFORMER IGNITRONS DC. ^ RFCORDFR POTENTIOMETER TEMPERATURE FEEDBACK 1 1 1 1 SF 1 =ECIMEN
vy^ •J^j)^ </\^-^vi^. s/^/f\/\/\^
/ v V w v A
PEAK POSITION Sa.ECTOR SWITCH
[ A A A A A A A A A A A / W V ^ A
-FEEDBACK LINEAR POTENTIOMETER
SERES RESISTOR AMPUTIJOÉ LMrTtNG NETWORK 10 TURN neucAL POTENTIOMETER - + 50 VOLTS , to TURN HEUCAL POTENTIOMETER O U T P U T T O H E A T U N I T [ E R R O R ]
FIG. 8. C I R C U I T DIAGRAM OF SIMULATOR.
E 200 IOC O \L — — —
-1
!\ ^ ( — \ \ 1 _- : i ! 1 11
1
^S
k
• ^ ^ ï ^ ? * ^ EXPERIMENTAL SfMULATED1
— 1 50 55 Tins Eacs)FIG to. OWPARISOrJ OF EXPERIMENTAL AND SIMULATED CYaES.
50 55 Time (Sees) I IG, 11. COMPARISON OF EXPERIMENTAL AND SIMULATED CYaES.
1200 1100 . 900 u s a 800 0 5 600 u 1 (00 300 200 no i J fï
T
' \ \ \ 1 N^ J l ! • 2C • ? ..,,,. : 3 ) - • •• -j r-—• T! Distance from Fusion Boundary 3 - " C 3 0 11" SIMUtATED 1 1 ! i • -3 ^ • -J -J ! sh ?- J
—a Tim* (Sees)E3 E l / ' / ' / ^ * y \ i / 1 / I / / { ^ i I / A / ' ' / ' / / I / I ' / 1/ / ' / I ' y / / I ' / / / / ' I I / / ' / / ' I ' / / / / I t ' / //y I I ' if/At - T - --} - -T h h T^ WIPER B POSITDN FIG. A 2. RESISTANCE i i TAP POSITION (WIPER B ) FIG A3.
6 8 TAP POSITION FIG. fKL. RESBTANCE TAP POSITION FIG. A 5.
® 1 0 0 ^ 5 G n ® 50 n. ® lOOü SOxi. @ 50X1 ® 100x1. Son.
