稀有金属(英文版) 2015,34(11),776-782

收稿日期：5 June 2015

基金：financially supported by the National High Technology Research and Development Program of China (No. 2007AA03Z506);the National Basic Research Program of China (No. 2015CB654803);

Ni segregation and thermal stability of reversed austenite in a Fe–Ni alloy processed by QLT heat treatment

Tao Pan Jing Zhu Hang Su Cai-Fu Yang

Division of Structural Materials, Central Iron and Steel Research Institute

School of Materials Science and Engineering, Tsinghua University

Abstract：

High-resolution transmission electron microscopy(HRTEM) and X-ray diffraction(XRD) were used to investigate Ni segregation and thermal stability of reversed austenite(RA) in a Fe–Ni alloy processed by quench–lamellarize–temper(QLT) heat treatment. The results show that the 77 K impact energy of the alloy increases with RA content increasing. As an austenite-stabilizing element, Ni is found to segregate in RA, though Ni is not evenly distributed within RA. The amount of segregations increases near the boundary(twice as high as the balanced content)and decreases to some extent in the center of the RA regions. Ni concentration in matrix near the boundary is lower than that in matrix far from the boundary because of Ni atom transportation from a to c near the boundary. RA in this alloy has high heat and mechanical stability but is likely to lose its stability and transform to martensite when a mechanical load is applied at ultralow temperatures(77 K), which induces plasticity.

Keyword：

Reversed austenite; Quench–lamellarize–temper; Component segregation; Heat stability; Mechanical stability;

Author： Tao Pan e-mail: taopan@vip.sina.com;

Received： 5 June 2015

1 Introduction

The continuously increasing demand for clean energy sources stimulates the development of the worldwide liq- uefied natural gas (LNG) industry over the past decade [1, 2]. Fe–Ni alloys (such as Fe–36Ni and Fe–9Ni) are important cryogenic materials in LNG tanks and LNG ships, which are subjected to the extreme cold temperatures of LNG (-165 °C) and require excellent cryogenic toughness to meet engineering safety standards [3]. Tra- ditional commercial Fe–9Ni alloys, produced using quench–temper heat treatments, are provided with excel- lent cryogenic toughness under modern metallurgic con- ditions [4, 5]. The cryogenic toughness of the e-Ni alloy is closely associated with the content and thermal stability of reversed austenite (RA) [6–8]. Thus, a high content of thermally stable RA usually yields Fe–9Ni alloy with a high toughness. In recent decades, researches are often focused on the quench–lamellarize–temper (QLT) process for preparing body-centered cubic (BCC) Fe–Ni alloy (such as Fe–9Ni alloy) [9]. An intercritical quenching (L- step) shall be added at a temperature between A_c1(trans- formation point of c phase to carbide phase) and A_c3(transformation point of c phase to a phase), prior to the tempering step, to enhance notch toughness and improve cryogenic embrittlement resistance.

Quenching at a temperature higher than A_c3yields lath martensite with a high density of dislocations and an extremely small quantity of retained austenite. During L-step, the Fe–Ni alloy decomposes into a mixture of tempered martensite and austenite, with diffusion of Ni and Mn atoms into austenite. Most of the austenite transforms to martensite during cooling because of insufficient alloy enrichment (component segregation). The resulting microstructure is a ‘‘dual-phase’’ structure of low-alloy laths (original martensite) and high-alloy laths (the fresh martensite) [10]. When the tempering (T) step is performed at a temperature actually higher than A_c1point of high- alloy laths, higher alloy austenite nucleates along the boundaries of high-alloy laths, resulting in a dense distri- bution of RA. To toughen Fe–Ni alloy, RA must be ther- mally stable [11]. Stability of RA is achieved by strong solute segregation of Ni and Mn element [12, 13]. QLT treatment was originally used to reduce the amount of costly Ni required in the alloy. For this reason, Fe–5.5Ni cryogenic alloy was studied with the aim of reaching toughness similar to that of Fe–9Ni alloy. However, QLT treatment has not been carefully researched for the purpose of excavating cryogenic toughness of the Fe–9Ni alloy [14]. Thus, in this paper, the component segregation and thermal stability of RA in Fe–9Ni alloy produced using QLT treatment were investigated.

2 Experimental

The commercial Fe–9Ni alloy used for Chinese LNG tanks withchemicalcomposition9 wt%Ni–0.64 wt%Mn– 0.3 wt%Si–0.06 wt%C–Fe was studied. The A_c1and A_c3temperatures of this alloy are 610 and 705 °C, respectively. The heat treatment applied was 800 °C soaking for 1 h and quenching, followed by an L-step at a temperature between 600 and 720 °C for 1 h and quenching, and finally tem- pering at 560 °C for 2 h. The standard QLT treatment used in this study was 800 °C (Q) ? 660 °C (L) ? 560 °C (T).

Charpy V-notched impact tests were performed at -196 °C (77 K) using a JB50 impact tester. The specimens were put into liquefied nitrogen for at least 10 min, and then, the impact tests were conducted for 3 s after being removed from the nitrogen. Standard tensile speci- mens were machined with working part dimensions of U 10 mm 9 50 mm and tested on a MTS810 tester at room temperature (RT, 20 °C), -120, -165, and -196 °C. Extra tensile test started and stopped at a retained strain of 0.05, 0.10, or 0.15.

The fine microstructure of the experimental materials was observed using a FEI Tecnai G2.0 high-resolution transmission electron microscope (HRTEM), and the microzone alloy composition of the RA was tested using an energy-dispersive X-ray spectroscopy (EDX) analyzer with a resolution of 5–10 nm. Tensile and impact specimens were used to measure RA content by X-ray diffractometer (XRD, Philips APD-10X) combined with Rietveld whole- spectrum over-fitting analysis. Computational kinetics is often used for the simulation of microstructure evolution and materials design [15]. The commercial material cal- culation software Thermo-Calc/DICTRA was used for dynamic simulation of the QLT heat treatment of the alloy using the parameters in Ref. [16], and results were com- pared with experimental observations.

3 Results and discussion

3.1 Relationship between toughness and RA

Variations of the cryogenic toughness of Fe–9Ni alloy and RA content as a function of L-step temperature of QLT process are shown in Fig. 1. When the L-step temperature is 640–680 °C, RA content is as high as 10 vol%–14 vol% and the 77 K impact energy is above 200 J. When L-step temperature is -720 °C or 600–620 °C, RA content decreases to approximately 8 vol% and the impact energy drops by approximately 20 %. The results indicate that 77 K impact energy of Fe–9Ni alloy increases with RA content. HRTEM images of RA in Fe–9Ni alloy after QLT treatment are shown in Fig. 2. RA forms dense island- and bar-like shapes in the alloy as pointed by the arrows in Fig. 2a, b.

3.2 Microzone composition of RA

Composition scanning data for RA and matrix obtained using HRTEM are shown in Fig. 3. Ni and Mn peaks and Fe troughs are observed at RA locations, indicating enrichment of austenite-stabilizing elements, such as Ni and Mn, in RA. Further energy spectrum analysis shows that the chemical composition distributes unevenly within a single RA region. All of the energy spectrum data show that Mn and Ni concentrations are higher near the boundary but lower in the center.

Quantified composition measurements of Ni and Mn in RA and matrix regions were made. The positioning scheme used is shown in Fig. 4, where Points 3, 5, and 7 denote RAnear a/c boundary, Point 4 indicates the middle of RA region, Points 2 and 6 denote the matrix near a/c boundary, and Point 1 indicates the matrix far away from RA. Theresults in Fig. 5 show that Ni concentration reaches as high as 21 wt% on one RA boundary and 18 wt% on the opposite boundary of the same RA region. However, Ni concentra- tion in the middle of designated RA region drops to 13.6 wt%, 20 wt%–30 wt% lower than that at the boundary position but still higher than bulk composition (9 wt%) of the material. Ni concentration of the matrix close to the RA drops dramatically to only 4.0 wt% and that in the regionfar away from the RA recover to the bulk composition (approximately 9 wt%) of the materials. Mn concentration follows a similar trend as Ni concentration. The highest Mn concentration is approximately 2.1 wt%, with an enrich- ment coefficient (defined as the ratio of local and bulk composition of alloy elements) of more than 3.0.

Fig. 1 77 K impact energy and RA content as a function of intercritical quenching temperatures

Fig. 2 HRTEM images and diffraction pattern of RA in Fe–9Ni alloy after QLT treatment: a bright field image, b dark field image, c diffraction pattern, and d diffraction pattern indexing

Fig. 4 Positioning scheme used for chemical composition measurement

Fig. 3 HRTEM image a and corresponding microzone line scanning of RA and matrix regions: b Ni, c Mn, and d Fe

Fig. 5 Elemental distribution in RA and matrix regions (MR)

3.3 Thermal stability of RA

The heat stability of RA was studied by measuring the variation of RA content in the alloys before and after soaking at 77 K (liquefied nitrogen temperature) for 10 h. The results are shown in Fig. 6. RA content after 77 K soaking does not change significantly with L-step temper- ature. This suggests that RA in the alloy generated by QLT process has a high degree of heat stability.

The variation of RA content with plastic strain at dif- ferent temperatures is shown in Fig. 7. The results show that RA is stable under mechanical loading at RT. How- ever, mechanical loading at low temperatures reduces RA content; RA reduction increases with temperature decreasing. For example, 15 % strain at -196 °C wasmeasured, which reduces RA content by 77 vol%, resulting in a final RA content of 3.1 vol %. This suggests that RA has excellent mechanical stability at RT, but RA would transform more or less if the material was strained at a temperature below -120 °C. When deformed by 15 % at -196 °C, RA loses its thermal stability and transforms into martensite.

Fig. 6 Effect of super-cooling on RA content (cooling at 77 K)

Fig. 7 RA content as a function of plastic strain at various temperatures

XRD was used to measure RA content closely along the fracture surface of 77 K impact specimens. The results are shown in Fig. 8. Impact loading drives RA content to below 1 vol%, which is even much lower than that after low-temperature tensile deformation.

3.4 Discussion

Introducing the L-step between normal quenching and tempering has a significant impact on the fine microstruc- ture of the materials. Composition profiles from single lath models treated with typical QLT process simulated using DICTRA software are shown in Fig. 9. The calculated results show that there are significant concentration fluc- tuations at several positions within a single lath after QLT treatment. The peak C concentration is approximately0.4 wt%, seven times as high as the bulk composition, and peak Ni concentration is approximately 18 vol%, two times as high as the balanced composition. High levels of Ni and C segregation in the austenite present a high degree of thermal stability. The simulation results suggest that Ni is not evenly distributed within a single RA region; Ni con- centration is higher in a/c boundary and lower within RA region. The simulation results agree well with the experi- mental results shown in Figs. 3 and 5.

Fig. 8 RA content before and after 77 K impact loading

Fig. 9 Composition profile after a typical QLT treatment (DICTRA simulation) for a Ni and b C

Table 1 Tensile properties of Fe–9Ni alloy at different temperatures  下载原图

Table 1 Tensile properties of Fe–9Ni alloy at different temperatures

During L- and T-steps of QLT process, C, Ni, and Mn segregate from the matrix to the element-enriched zones by atom diffusion. The short-distance diffusion of Ni and Mn, which is governed by kinetic energy, results in higher concentrations of these elements on RA boundaries and a relative trough between the boundaries of a given RA region. However, C diffuses more easily and reaches the equilibrium concentration of austenite. RA has an internal C peak because of the Ni trough. Formation of Ni- and Mn- enriched regions is dependent on atom transportation near a/c boundaries. This leads to rather low Ni and Mn con- centrations in the matrix near boundaries. Ni and Mn concentrations of matrix region far away from a/c boundary are close to the bulk composition.

Tensile properties of experimental materials at different temperatures are shown in Table 1 and Fig. 10. Tensile strength increases with temperature decreasing. This is consistent with the general mechanical behavior of BCC- structured materials. Interstitial impurity atoms generate nonspherical distortion of BCC lattice, resulting in pinning up of both helical and edge dislocations. Lowering tem- perature increases the critical resolved shear stress, which unpins dislocations. As a result, the fracture mode changes from shear to cleavage fracture and the material becomes more brittle and less plastic. However, uniform elongation of experimental material increases with temperature decreasing. Uniform elongation at -196 °C increases to as high as 21.5 %, which is significantly higher than that at RT. And yield ratio declines unexpectedly and the material exhibits improved toughening performance.

The improved ‘‘toughening’’ behavior at lower temper- atures is a result of RA transformation during tensile pro- cess. As shown in Fig. 7, tensile loading at low temperatures makes RA less stable and leads to a gradual transformation. Figure 11 shows HRTEM images and diffraction pattern of ‘‘RA’’ after -196 °C tensile loading, which is proved to already transform into martensite by thediffraction pattern indexing. These results demonstrate that sustaining transformation of RA would provide tensile specimens with continuous work hardening and prolong the uniform elongating process. Continuous work hardening increases the gap between yield strength and ultimate tensile strength, resulting in a lower yield ratio. The pro- longed elongating process enhances uniform elongation. The performance of RA after low-temperature loading is described as the transformation-induced plasticity (TRIP) effect, which is proposed to understand the high cryogenic toughness produced by BCC-structured Fe–Ni alloys, such as Fe–5.5Ni and Fe–9Ni [7, 17]. Because the amount of transformed RA at -196 °C is much larger than that at other temperatures, it is reasonable to expect that the material will exhibit enhanced ‘‘toughening’’, including lower yield ratio and higher uniform elongation. It is noted that the yield ratio of the QLT-treated Fe–9Ni alloys is no more than 0.81 at -165 °C, which is the service temper- ature for LNG tanks. This means that the alloy processed with QLT heat treatment would be safe to use for a LNG tank.

Fig.10 Mechanical properties versus testing temperature:a strength and b plasticity

Fig. 11 HRTEM images and diffraction pattern of ‘‘RA’’ after low- temperature loading: a bright field image, b dark field image, c diffraction pattern, and d diffraction pattern indexing

4 Conclusion

The chemical composition and stability of RA were investigated. Experimental results show that the elements in RA structure of Fe–9Ni alloy are not evenly distributed. The highest Ni and Mn concentrations in RA are observed near the boundary between RA and matrix, while the matrix near the boundary has the lowest Ni and Mn con- centrations. RA in the alloy has excellent heat and mechanical stability. However, RA loses its stability and transforms into martensite after mechanical loading at ultralow temperatures. TRIP effect of RA under low-tem- perature loading is an important reason for excellent cryogenic toughness of Fe–9Ni alloy.