熔焊原理 94ReducingSolidificationCracking

1、9.4 Reducing Solidification Cracking9.4.1 Control of Weld Metal CompositionWeld metals of crack-susceptible compositions should be avoided. In autogenous welding no filler metal is used, and the weld metal composition is determined by the base-metal composition. To avoid or reduce solidification cra

2、cking, base metals of susceptible compositi ons should be avoided. Whe n a base metal of a crack-susceptible composition has to be welded, however, a filler metal of a proper composition can be selected to adjust the weld metal composition to a less susceptible level.A. Aluminum Alloys According to

3、Figure 9.13, an AI-3% Cu alloy can be rather crack susceptible during welding. If the Cu content of the base metal is raised to above 6%, solidification cracking can be significantly reduced. In fact, 2219 aluminum, one of the most weldable Al-Cu alloys, contains 6.3% Cu. When a filler metal is used

4、, the weld metal composition is determined by the composition of the base metal, the composition of the filler metal, and the dilution ratio. The dilution ratio, as mentioned previously, is the ratio of the amount of the base metal melted to the total amount of the weld metal. Aga in using Al-3% Cu

5、as an example, the weld metal Cu content can be in creased by using 2319 filler metal, which is esse ntially anAl-6.3% Cu alloy. If the joint desig n and heat in put are such that the dilutio n ratio is low, the weld metal copper content can be kept sufficiently high to avoid solidification cracking

6、. Figure 9.25 shows the approximate dilution in three typical joint desig ns.BO % filler meiar20 %meldifi(ttr mrlaitost mua Ifi 11 et ir ei ql tose me To ISquQFt*6idFigure 9.25 Approximate diluti on of three weld joi nts by base metal in alumi num weldi ng.Table 9.2 is a guide to choice of filler me

7、tals for minimizing solidification crack ing in welds of high-stre ngth alu minum alloys. Experime ntal data such as those in Figure 9.26 from solidification cracking testing by ring casting can also be useful.Table 9.2 Guid to Choice of Filler Metals for Minimizing Solidification Crackingin Welds o

8、f High-Stre ngth Alumi num AlloysBase Metals2111) (Al-C u5000 (Al Mg)6000 (AJ Mg Si7U00 (Al-Zn-Mn)7(MX) (Al-n-M*-Cu)7000(AL-Zn-Mg-CuM&535653565?5&TOGO(Al-Zn-Mg)NR行 00050002000(Al-Mg-Si) (Al-Mg) (Al-Cu)NRNR4 I4i3565356414_o5556S55651 S351H34(M346435.VGl 2 $4 Cflrt,ih ih ifiNEi. not tcetHTirti.Tid.I l

9、i iris ihjt rccuiinnieiiLl Idler choice tnr ninnv dntikc 汁 tinnssho 由1、e tron hl lei RielaJ uuinli-rvJ. 7 6? cm4 10 cm. 4inEs 亶-UJn5 1? 7cftTi 5 In6 1 $丄網(wǎng) in z I? raom 7ifl 耳上口左口n,4Jn 孕?2.*a om, SinSilicon. %Silicon. %40S4 言 2 Fn 旨一一 L.4 cm, 12 hFigure 9.26 Solidification cracking susceptibility of

10、aluminum alloys:Wiglil % Co peer(a) Al-Mg-Si; (b) Al-Cu-Si (c) Al-Mg-Cu.Minor allo ying eleme nts have also bee n found to affect the solidificati on crack ing susceptibility of aluminum alloys. For example, the Fe-Si ratio has been found to significantly affect the solidification cracking susceptib

11、ility of 3004 and other Al-Mg alloys; therefore, proper control of the content of minor alloying elements can be importa nt in some materials.B. Carbon and Low-Alloy Steels The weld metal manganese content can sig nifica ntly affect solidificati on crack in g. It is ofte n kept high eno ugh to en su

12、re the formation of MnS rather than FeS. This, as described previously, is because the high melting point and the globular morphology of MnS tend to render sulfur less detrimental. Figure 9.27 shows the effect of the Mn-S ratio on the solidification crack ing tendency of carb on steels. At relativel

13、y low carb on levels the solidificati on cracking tendency can be reduced by increasing the Mn-S ratio. However, at higher carb on levels (i.e., 0.2-0.3% C), i ncreasi ng the Mn-S ratio is no Ion ger effective. In such cases loweri ng the weld metal carb on con ten t, if permissible, is more effecti

14、ve.3020100,10 012 0J4 0 16Carbon content, %5040Figure 9.27 Effect of Mn-S ratio and carb on content on solidificati on crack ing susceptibility ofcarb on steel weld metal.One way of loweri ng the weld metal carb on content is to use low-carb on electrodes. I n fact, in weldi ng high-carb on steels o

15、ne is ofte n required to make the first bead (i.e., the root bead) with a low-carb on electrode. This is because, as show n in Figure 9.28, the first bead tends to have a higher diluti on ratio and a higher carb on content tha n subseque nt beads. A high carb on content is un desirable because it pr

16、omotes not only the solidificati on crack ing of the weld metal but also the formati on of brittle martensite and, hence, the postsolidification cracking of the weld metal. Therefore, in weldi ng steels of very high carb on contents (e.g., greater tha n 1.0% C), extra steps should be take n to avoid

17、 in troduci ng excessive amounts of carb on from the base metal into the weld metal. As show n in Figure 9.29, one way to achieve this is to butter” the groove faces of the base metal with auste nitic sta ini ess steel (such as 25-20 sta ini ess) electrodes before weld ing. In weld ing cast irons, p

18、ure ni ckel electrodes have also been used for buttering. In any case, the surface layers remain in the ductile auste nitic state, and the weld can the n be completed either with sta ini ess electrodes or with other cheaper electrodes.Figure 9.28 Schematic sketch of multipass weldi ng. Note that the

19、 root pass has the highest diluti on ratio.Figure 9.29 Butteri ng the groove faces of very high carb on steel with a 310 sta ini ess wire steel before weldi ng.C. Nb-Bearing Stainless Steels and SuperalloySThe C-Nb ratio of the weld metal can affect the susceptibility to solidification cracking sign

20、ificantly. A high C-Nb ratio can reduce solidification cracking by avoiding the low-temperature L丫 + Laves reacti on, which can wide n the solidificatio n temperature ran ge.D. Austenitic Stainless Steels As men ti oned previously, it is desirable to maintain the weld ferrite content at a level of 5

21、-10% in order to obtain sound welds. The qua ntitative relati on ship betwee n the weld ferrite content and the weld metal compositi on in auste nitic sta ini ess steels has bee n determ ined by Schaeffler, DeLong, Kotecki, Balmforth and Lippold, and Vitek et al. These constitution diagrams have bee

22、n shown previously in Chapter 7. Alloying elements are grouped into ferrite formers (Cr, Mo, Si, and Cb) and austenite formers (Ni, C, and Mn) in order to determine the corresponding chromium and nickel equivalents for a given alloy.Example: Consider the welding of a 1010 steel to a 304 stainless st

23、eel. For convenien ce, let us assume the diluti on ratio is 30%, half from 304 sta in less steel and half from 1010 steel, as shown in Figure 9.30. Estimate the ferrite content and the solidification cracking susceptibility of a weld made with a type 310 electrode that contains 0.12% carb on and a w

24、eld made with a type 309 ELC (extra low carb on) electrode that contains 0.03% carb on.ElectrodeFigure 9.30 Weldi ng 304 sta in less steel to 1010 carbon steel.For the weld made with the 310 sta ini ess steel electrode, the weld metalcompositi on can be calculated as follows:Eleme ntElectrodeX70%Typ

25、e304X15%1010SteelX15%Weld MetalCr26.018.218.02.70020.9Ni21.014.78.01.20015.9C0.120.0840.050.00750.100.0150.1065Mn1.751.232.00.300.40.061.59Si0.40.280.20.030.31According to the WRC-1992 diagram shown in Figure 7.11, the chromium and nickel equivalents of the weld metal are as follows:Chromium equival

26、e nt = 20.9Nickel equivale nt = 15.9 + 35 X0.1065 = 19.6Based on the diagram, the weld metal is fully austenitic and, therefore, is susceptible to solidificati on crack in g. If the 309 ELC electrode is used, the weld metal compositi on can be calculated as follows:Eleme ntElectrodeX70%Type304X5%101

27、0SteelX15%Weld MetalCr2416.818.02.70019.5Ni139.18.01.20010.3C0.030.0210.050.00750.100.0150.0435Mn1.981.392.00.300.40.061.75Si0.40.28000.20.030.31Therefore, the chromium and n ickel equivale nts of the weld metal are Chromium equivale nt = 19.5Nickel equivale nt = 10.3 + 35 x0.0435 = 11.8According to

28、 the WRC-1992 diagram, the weld metal now is austenitic with a ferrite nu mber of about 8 and, therefore, should be much more resista nt to solidificati on crack ing.It should be emphasized that neither the constitution diagrams nor the magnetic measurements of the weld metal ferrite content (such a

29、s those determined by Magne-Gage readings) reveal anything about the weld metal solidification. In fact, the primary solidification phase and the quantity of ferrite at high temperatures(i.e., during solidification) are more important than the amount of ferrite retained in the room temperature micro

30、structure in determining the sensitivity to solidification cracking. Also, the amount of harmful impurities such as sulfur and phosphorus should be considered in determining the weldability of a material; a material with a higher ferrite content can be more susceptible to solidificati on crack ing t

31、ha n ano ther material with a lower ferrite content if the impurity level of the former is also higher. The cooling rate during solidification is another factor that the constitution diagrams fail to recog nize (Chapter 7).9.4.2 Control of Solidification StructureA. Grain Refining As mentioned previ

32、ously, welds with coarse columnar grains are often more susceptible to solidification cracking than those with fine equiaxed grains.It is, therefore, desirable to grain refine the weld metal. In fact, both 2219 aluminum and 2319 filler metal are designed in such a way that they not only have a non-c

33、rack-sensitive copper content but also have small amounts of grain refining age nts such as Ti and Zr to mini mize solidificati on crack ing. Dudas and Colli ns produced grain refining and eliminated solidification cracking in a weld made with an Al-Zn-Mg filler metal by adding small amounts of Zr t

34、o the filler metal. Garland has grain refined welds of aluminum-magnesium alloys by vibrating the arc during welding, thereby reduc ing solidificati on crack ing.B. Magnetic Arc Oscillation Magnetic arc oscillation has been reported to reduce solidification cracking in aluminum alloys, HY-80 steel,

35、and iridium alloys. Kou and Le studied the effect of magnetic arc oscillation on the grain structure and solidification cracking of aluminum alloy welds. As already shown in Figure 5.30, tran sverse arc oscillati on at low freque ncies can produce alter nati ng colu mnar grains. Figure 9.31 dem on s

36、trates that this type of grain structure can be effective in reduc ing solidification cracking. As illustrated in Figure 9.32, columnar grains that reverse their orie ntati on at regular in tervals force the crack to cha nge its direct ion periodically, thus making crack propagation difficult. Figur

37、e 9.33 shows the effect of the oscillation frequency on the crack susceptibility of 2014 aluminum welds. As shown, a minimum crack susceptibility exists at a rather low frequency, where alternating grain orie ntati on is most pronoun ced. This freque ncy can vary with the weldi ng speed.As shown in

38、Figure 9.34, arc oscillation at much higher frequencies than 1Hz, though in effective in 2014 alumi nu m, is effective in 5052 alumi num.ALLOY 2Q4-1*5 古140 TRANSVERSE OSCILLATION OSCILLATION-OFigure 9.31 Effect of tran sverse arc oscillati on (1Hz) on solidificatio n crack ing in gas-t un gste n arc

39、 welds of 2014 alumi num.CURVED CCLLMNAHGRAINSb STRAIGHT CnACK RATHFigure 9.32 Schematic sketches show ing effect of arc oscillati on on solidificatio n crack ing.2014 aluminumI |transvrsA oscillationjQ51035Frequency. HzE 150肓100 50C 0Figure 9.33 Effect of oscillation frequency on solidification cra

40、cking in gas-tungsten arc welds of 2014 alumi num.Frequency HzFigure 9.34 Effect of arc oscillation frequency on the solidification cracking in gas-tungsten arc welds of 5052 alu minum.Figure 9.35 Grain structure of gas-t un gste n arc welds of 5052 alumi num:(a) no arc oscillation; (b) 20Hz transve

41、rse arc oscillation.As shown in Figure 9.35, this is because grain refining occurs in alloy 5052 welds at high oscillation frequencies. Heterogeneous nucleation is believed to be mainly responsible for the grain refining, since a 0.043wt% Ti was found in the 5052 alu minum used.9.4.3 Use of Favorabl

42、e Welding ConditionsA. Reducing Strains As mentioned previously in Chapter 1, the use of high-intensity heat sources (electron or laser beams) significantly reduces the distortion of the workpiece and hence the thermally induced strains. Less restraint and proper preheat ing of the workpiece can als

43、o help reduce stra ins.Dyatlov and Sidoruk and Nikov found that preheating the workpiece decreased the magnitude of strains induced by welding. Sekiguchi and Miyake reduced solidification cracking in steel plates by preheating. Hernandez and North positioned additional torches behind and along the s

44、ide of the welding head and inhibited solidification cracking in aluminum alloy sheets. It was suggested that the local heat ing decreased the amount of plastic stra ining result ing from the weld ing operati on and produced a less stressful situati on behi nd the weld pool.B. Improving Weld Geometr

45、y The weld bead shape can also affect solidification crack ing. When a con cave sin gle-pass fillet weld cools and shri nks, the outer surfaceis stressed in tension, as show in Figure 9.36a. The outer surface can be con sidered as being pulled toward the toes and the root. However, by making the outer surface convex, as shown in Figure 9.36b, pulling toward the root actually compressesthe outer surface and offsets the tension ca