熔焊原理 71Ferrite to AusteniteTransformationinAusteniticStainlessSteelWelds

1、7.1 Ferrite-to-Austenite Transformation in Austenitic Stainless SteelWelds7.1.1 Primary Solidification ModesThe welds of austenitic stainless steels normally have an austenite (fee) matrix with varying amounts of ferrite (bcc). A proper amount of -ferrite in austenitic stainless steel welds is essen

2、tial-too much rferrite ( NO vol %) tends to reduce the ductility, tough ness, and corrosi on resista nee, while too little rferrite (5 vol %) can result in solidificati on crack ing.A. Phase DiagramClW 和 M 4C W 0070 &O NiVYKtglil pn:wTlAgai niahalFigure 7.1 The Fe-Cr-Ni ternary system: (a) liquidus

3、surface; (b) solidus surfaceFigure 7.1 shows the ternary phase diagram of the Fe-Cr-Ni system. The heavy curved line in Figure 7.1a representsthe trough on the liquidus surface, which is called the line of twofold saturati on. The line decli nes from the bi nary Fe-Ni peritecticreaction temperature

4、to the ternary eutectic point at 49Cr-43Ni-8Fe.Alloys with a composition on the Cr-rich (upper) side of this line have ferrite as the primary solidification phase, that is, the first solid phase to form from the liquid. On the other hand, alloys with a composition on the Ni-rich (lower) side have au

5、stenite as the primary solidification phase. The heavy curved 1ines on the solidus surface in Figure 7.1b more or less follow the trend of the liquidus trough and converge at the ternary eutectic temperature.Primary ausienlte (Primary fenilesoiidtication 1solidificationinterdendrttic Yi ferritevermi

6、cularferritelath/ yor ferrite(b2)ncreasmg Ni increasing Cr(d)Figure 7.2 Schematics show ing solidificati on and postsolidificati on tran sformatio n in Fe-Cr-Ni welds: (a) in terde ndritic ferrite; (b) vermicular ferrite; (c) lathy ferrite; (d) vertical sect ion of tern ary-phase diagram at approxim

7、ately 70% Fe.The development of weld metal microstructure in austenitic stainless steels is explained in Figure 7.2. The weld metal ferrite can have three different types of morphology: in terde ndritic (Figure 7.2a), vermicular (Figure 7.2b), and lathy (Figure 7.2c). Figure 7.2d shows a schematic v

8、ertical (isoplethal) secti on of the ternary phase diagram in Figure 7.1, for instanee, at 70 wt % Fe and above 1200 C. This has also bee n called a pseudo-b inary phase diagram. The apex (po int 1) of the three-phase eutectic tria ngle (L+ 7+ S) corresp onds to the in tersectio n betwee n the verti

9、cal sect ion and the heavy curved line in Figure 7.1a.The two lower corners (po ints 2 and 3) of the triangle, on the other hand, correspond to the intersections between the vertical secti on and the two heavy curved lines in Figure 7.1b.B. Primary Austenite For an alloy on the Ni-rich (left-hand) s

10、ide of the apex of the three-phase eutectic triangle, austenite ( 丫 )s the primary solidification phase. Thelight dendrites shown in Figure 7.2a are austenite, while the dark particles between the primary dendrite arms are the ferrite that forms when the three-phase triangle is reached during the te

11、rminal stage of solidification. These are called the interdendritic ferrite. For dendrites with long secondary arms, interdendritic ferrite particles can also form between secondary dendrite arms.C. Primary Ferrite For an alloy on the Cr-rich (right-hand) side of the apex of the three-phaseeutectic

12、triangle, ferrite is the primary solidification phase. The dark den drites show n in Figure 7.2b areferrite. The core of the ferrite den drites, which forms at the beginning of solidification, is richer in Cr (point 4), while the outer portions, which form as temperature decreases, have lower chromi

13、um contents. Upon cooling into the (+ Ytwo-phase region, the outer portions of the dendrites having less Cr transform to austenite, thus leaving behind Cr-rich skeleton of 吝ferrite at the dendrite cores. This skeletal ferrite is called vermicular ferrite. In addition to vermicular ferrite, primary f

14、errite dendrites can also transform to lathy or lacy ferrite upon cooling into the (狂 Y two-phase region, as shown in Figure 7.2c.D. Weld Microstructure Figure 7.3a shows the solidification structure at the centerline of an autogenous gas-tungsten arc weld of a 310 stainless steel sheet, which conta

15、ins approximately 25% Cr, 20% Ni, and 55% Fe by weight. The composition ison the Ni-rich (left) side of the apex of the three-phase eutectic triangle, as shown in Figure 7.4a, and solidification occurs as primary austenite. The microstructure consists of austenite dendrites (light etching; mixed-aci

16、ds etchant) and interdendritic 咅ferrite (dark etch ing; mixed-acids etcha nt) betwee n the primary and sec on dary dendrite arms, similar to those shown in Figure 7.2a.Figure 7.3b, on the other hand, shows the solidification structure at the centerline of an autogenous gas-tungsten arc weld of a 309

17、 stainless steel sheet, which contains approximately 23 wt% Cr, 14 wt% Ni, and 63 wt % Fe. The composition lies just to the Cr-rich side of the apex of the three-phase eutectic triangle, as shown in Figure 7.4b, and solidifies as primary 咅ferrite. The microstructure consists of vermicular ferrite (d

18、ark etching; mixed-acids etchant) in an austenite matrix (light etching; mixed-acids etchant) similar to those shown in Figure 7.2b. In both welds columnar dendrites grow essentially perpendicular to the teardrop-shaped pool boundary as revealed by the columnar dendrites.Kou and Le quenched welds du

19、ring welding in order to preserve the as-solidified microstructure, that is, the microstructure before post-solidification phase transformations. For stainless steels liquid-tin quenching is more effective than water quenching because steam and bubbles reduce heat transfer. With the help of quenchin

20、g, the evolution of microstructure during welding can be better studied. Figure 7.5 shows the ferrite dendrites (light etching; mixed-chloride etchant) near the weld pool of an autogenous gas-tungsten arc weld of 309 stainless steel, quenched in during welding with liquid tin before the 5ytransforma

21、tion changed it to vermicular ferrite like that shown in Figure 7.3b. Liquid-tin quenching was subsequently used by other investigators to study stainless steel welds.(a) 310 sta in less steel; (b) 309 stai nless steel. Mag ni fication 190302010271775 IS 2Swt%Cr0-dn6ac!Eal(:73嗚 Fe I22 122 wt%NiFigur

22、e 7.4 The Fe-Cr-Ni pseudo-b inary phase diagrams: at 55 wt % Fe; (b) at 63wt % Fe; (c) at 73 wt % Fe.Figure 7.5 Liquid-tin quenched solidification structure near the pool of an autogenous gas-tungsten arc weld of 309 stainless steel. Magnification 70 X Mixed-chloride etchant.7.1.2 Mechanisms of Ferr

23、ite FormationInoue et al. studied vermicular and lathy ferrite in autogenousGTAW of austenitic stainless steels of 70% Fe with three different Cr-Ni ratios. It was found that, as theCr-Ni ratio in creases, the ratio of lathy ferrite to total ferrite does not cha nge sig nifica ntly eve n though both

24、 in crease. A schematic of the proposed formati on mechanism of vermicular and lathy ferrite is shown in Figure 7.6. Austenite first grows epitaxially from the unmelted austenite grains at the fusion boundary, and 咅ferrite soon nucleates at the solidification front. The crystallographic orientation

25、relati on ship betwee n the ferrite and the auste nite determ ines the ferrite morphology after the postsolidificati on tran sformatio n. If the closed-packed pla nes of the ferrite are parallel to those of the austenite, the 3 丫 transformation occurs with a planar in terface, result ing in vermicul

26、ar ferrite. However, if the so-called Kurdjumov -Sachs orientation relationships, namely, (-110/(-111)記nd -1-11 -1-10 T, exist between the 吝ferrite and the austenite, the transformation occurs along the austenite habit plane into the ferrite dendrites. The resultant ferrite morphology is lathy, as s

27、hown in Figure 7.7. For the lathy ferrite to con ti nue to grow, the preferred growth directi on of both ferrite and auste nite must be alig ned with the heat flow directi on.of100 dract i on ofL iqu 4F u電一雯J乏r ARC ferrite/ number19IS1f?151412.Austenite plus ferrile 一1Chromiunn equiaienl -%Cr + %IVo

28、 4- 1.fi X %Si I i| I xVidmanstattenblocky Ai i 11|ceiluar F-lacv FIl vermicular FirbterceHulrATmassive A-interdendntic F intercellular rcelluar Acelldlar-dendrrtic Ai iiil i i i il i i I ilOr302&242210110101102103 it/1Electron-beam travel sceed, mm/sFigure 7.16 Electr on beam travel speed (cooli ng

29、 rate) versus compositi on map of microstructural morphologies of the seven alloys in Figure 7.15 (A and F denote austenite and ferrite, respectively). The solid lines indicate the regions of the four primary solidification modes, while the dashed lines represent the different morphologies resulting

30、 from postsolidification tran sformatio n from ferrite to auste nite.At very high welding speedssuch as 2000mm/s, however, the cooling rates are high and the alloys solidify in only the single-phase austenite mode (A) or the single-phase ferrite mode (F). An example of the former is the alloy 3 (abo

31、ut Fe-24.75Cr-16.25Ni) shown in Figure 7.17a. At the travel speed of 25mm/s3(2X10 C/s cooling rate) the substrate solidifies as primary austenite in the AF mode, with austenite cells and intercellular ferrite. At the much higher travel speed of 2000mm/s (1.5X0 C/s cooling rate) the weld at the top s

32、olidifies as primary austenite in the A mode, with much smaller austenite cells and no intercellular ferrite (cellular A). An example of the latter is alloy 6 (about Fe-27.5Cr-13.5Ni) shown in Figure 7.17b. At 25mm/s the substrate solidifies as primary ferrite in the FA mode, with blocky austenite i

33、n a ferrite matrix. At 2000mm/s the weld at the top solidifies as primary ferrite in the F mode, with ferrite cells alone and no austenite (cellular F). Figure 7.16 also dem on strates that un der high cooli ng rates an alloy that solidifies as primary ferrite at low cooling rates can change to prim

34、ary austenite solidification. For in sta nee, alloy 4 (about Fe-25.5Cr-15.5Ni) can solidify as primary ferrite at low cooling rates (vermicular F) but solidifies as primary austenite at higher cooling rates (in tercellular F or cellular A).A no ther in teresti ng point see n in the same figure is th

35、at at high cooli ng rates alloy 5 can solidify in the fully ferritic mode and un dergoes a massive (diffusionless) transformation after solidification to austenite (massive A). Under very high cooling rates there is no time for diffusion to occur.3Figure 7.17 Microstructure of the low-cooling-rate s

36、ubstrate (2 x 10 C/s) and thehigh-cooli ng-rate electr on beam weld at the top: (a) alloy 3 in Figure 7.15; (b) alloy 6.B. Dendrite Tip Undercooling Vitek et al. attributed the change solidification mode, from primary ferrite to primary austenite, at high cooling rates to dendrite tip un dercooli ng

37、. Brooks and Thomps on expla ined this un der-cooli ng effect based on Figure 7.18. Alloy COsolidifies in the primary ferrite mode at low cooling rates. Under rapid cooling in laser or electron beam welding, however, the melt can undercool below the extended austenite liquidus (CLg), and it becomes

38、thermodynamically possible for the melt to solidify as primary austenite. The closer C o is to the apex of the three-phase triangle, the easier sufficient undercooling can occur to switch the solidificati on mode from primary ferrite to primary auste nite.Figure 7.18 Vertical sect ion of Fe-Cr-Ni ph

39、ase diagram show ing cha nge in solidificati on from ferrite to austenite due to dendrite tip undercooling.Kou and Le made autoge nous ga-un gste n arc welds in 309 sta ini ess steel, which has a composition close to the apex of the three-phase triangle, as shown in Figure 7.4b.At 2mm/s (5ipm) weld ing speed, primary ferrite was observed across the en tire weld (similar to that shown in Figure 7.3b). At a higher welding speed of 5mm/s (12ipm), however, primary austenite was observed along the centerline, as shown in Figure 7.19. Electro n probe micro-a nalysis (EPMA) revealed no