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Acta Metall Sin  2017, Vol. 53 Issue (7): 778-788    DOI: 10.11900/0412.1961.2016.00521
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Research on Austenite Transformation of FB2 Heat-Resistant Steel During Welding Heating Process
Kejian LI1,Zhipeng CAI1,2,3(),Yao WU4,Jiluan PAN1
1 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
2 State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
3 Collaborative Innovation Center of Advanced Nuclear Energy Technology, Beijing 100084, China
4 Tsinghua University Research Institute for Advanced Equipment, Tianjin 300304, China
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Abstract  

The improvement of steam parameters in fossil power plants requires the development of new kinds of 9% Cr martensitic heat-resistant steels, among which FB2 steel is a 100×10-6 (mass fraction) boron-containing steel and mainly used for manufacturing components with thick walls operating at high temperatures above 600 ℃. In the alloy system of martensitic heat-resistant steels, boron plays an important role in suppressing type IV crack of weld joints by the formation of heat affected zone (HAZ) with no fine grains in the normalized and intercritical zones, where there exhibit fine grains in conventional 9%Cr heat-resistant steels with no boron such as P91 steel. In this work, the formation process of HAZ in FB2 steel was investigated. The microstructures before and after thermal simulation were compared using OM and SEM. It was concluded that the austenization of FB2 steel at rapid heating rates (≥100 ℃/s) took place by shear mechanism, demonstrating austenite memory effect; while at slow heating rates (≤5 ℃/s), the austenization was by atom short range diffusion mechanism, without austenite memory effect. The special phase transformation of austenization is the main cause for the formation of HAZ with no coarsened grain in the overheated zone. Based on the previous results reported by other researchers, a preliminary model was proposed to describe how boron atoms change the austenite transformation type of FB2 steel during heating process, which developed the previous ideas about the phenomenon.

Key words:  FB2 steel      welding      martensitic transformation      austenite memory effect      boron     
Received:  21 November 2016     
Fund: Supported by Science and Technology Research Program of Shanghai Science and Technology Committee (No.13DZ1101502) and Tribology Science Fund of State Key Laboratory of Tribology of Tsinghua University (No.SKLT2015A02)

Cite this article: 

Kejian LI,Zhipeng CAI,Yao WU,Jiluan PAN. Research on Austenite Transformation of FB2 Heat-Resistant Steel During Welding Heating Process. Acta Metall Sin, 2017, 53(7): 778-788.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00521     OR     https://www.ams.org.cn/EN/Y2017/V53/I7/778

Fig.1  Schematic of cross-section of the weld joint and sampling method for microstructure observation
Rh / (℃s-1) tH / s Tp / ℃
960 1000 1250
1 0.5 × ×
5 0.5 × ×
100 0.5
30 ×
Table 1  Thermal cycle parameters of thermal simulation
Fig.2  OM images of HAZ in FB2 steel (HAZ—heat affected zone; Deq—equivalent diameter, the diameter of a circle occupying the same area with prior austenite grain in metallograph)

(a) macro metallograph of HAZ (b) magnification of zone I in HAZ (c) magnification of zone II in HAZ (d) magnification of base metal

Fig.3  Variation of expansion with temperature (T) under different thermal simulation conditions
Fig.4  OM images of FB2 steel before (a, c) and after (b, d) thermal simulations

(a, b) Rh=100 ℃/s, Tp=1000 ℃, tH=0.5 s (c, d) Rh=100 ℃/s, Tp=1250 ℃, tH=0.5 s

Fig.5  OM images of FB2 steel after different thermal simulations and tempered at 680 ℃ for 2 h (The red lines in Figs.5a and b show the prior austenite grain boundaries)

(a) Rh=100 ℃/s, tH=0.5 s, Tp=1000 ℃ (b) Rh= 100 ℃/s, tH=0.5 s, Tp=1250 ℃

Fig.6  SEM images of the same location in FB2 steel before (a) and after (b) thermal simulation with Rh=100 ℃/s, tH=0.5 s, Tp=1000 ℃ (The arrows show the straight boundaries inside martensite lathes)
Fig.7  SEM image of FB2 steel after thermal simulation with parameters Rh=100 ℃/s, tH=0.5 s, Tp=1000 ℃ (The arrows show the cylindrical interface surrounding partially melted precipitates)
Fig.8  OM images of FB2 steel after thermal simulation and tempered at 680 ℃ for 2 h (The inset in Fig.8a shows the magnified image, and the red lines show the prior austenite grain boundaries)

(a) Tp=1000 ℃, Rh=5 ℃/s, tH=0.5 s (b) Tp=1000 ℃, Rh=100 ℃/s, tH=30 s

Fig.9  Illustration of FB2 steel austenization process with Rh=100 ℃/s, Tp=1000 ℃, tH=30 s

(a) microstructure change of a certain martensite lath at rapid heating rates (b) microstructure change of a certain martensite lath with extended holding time or elevated peak temperatures (c) microstructure state after lath-like austenite break into equiaxed grains

Fig.10  Morphologies of FB2 steel with heating rate 5 ℃/s obtained by high temperature LSCM (The arrows and ellipses show the sites where austenite transformation took place)

(a) 279.21 s, 905.7 ℃ (b) 280.60 s, 914.8 ℃ (c) 281.80 s, 921.2 ℃ (d) 283.00 s, 927.7 ℃ (e) 284.19 s, 934.2 ℃ (f) 287.78 s, 951.9 ℃

Fig.11  Variation of free enthalpy (G) of martensite, α-Fe and γ-Fe with temperature (T1—a certain temperature at which austenite can transform to martensite spontaneously, Ms—martensite transformation starting temperature, T0—the temperature at which α-Fe and γ-Fe have the same free enthalpy, M—martensite, ΔGα→M—free enthalpy difference between α-Fe and M, ΔGγ→M —free enthalpy difference between γ-Fe and M, ΔGγα —free enthalpy difference between γ-Fe and α-Fe)
Tp / ℃ Rh / (℃s-1) Dislocation density in alstenite Ms / ℃
960 100 +++++ 391
1000 100 ++++ 381
1000 5 +++ 373
1000 1 ++ 364
1250 100 + 326
Table 2  Correspondence between Ms and dislocation density in austenite at different thermal conditions
Fig.12  Morphologies of FB2 specimen surface at two certain moments of 279.21 s (a) and 282.20 s (b) during heating process obtained by LSCM (The arrow and circle show the position where austenization took place)
Fig.13  Schematic for the distribution of B atoms in the lattice under rapid heating or as received condition (a) and slow heating condition (b)
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