Gas/Metal Reactions, Diffusion, and Phase Transformation during Heat Treatment of Steel: Gas/Metal Reactions, Diffusion, and Phase Transformation during Heat Treatment of Steel
Program Organizers: Daniel Baker, General Motors

Monday 8:00 AM
September 30, 2019
Room: C123
Location: Oregon Convention Center

Session Chair: Daniel Baker, General Motors

8:00 AM  
Age Hardening in Lightweight FeMnAl Steels with Nickel Additions: Laura Bartlett1; Michael Piston1; Krista Limmer2; Daniel Field2; 1Missouri University of Science & Technology; 2CC DEVCOM Army Research Laboratory
    High manganese and aluminum steels with additions of nickel have shown great promise as low density, ultra-high strength steels for automotive and structural applications. However, little is known about the role of thermomechanical processing and heat treatment on the stability of strengthening phases in this system. In this study, the effect of Ni addition on the age hardening kinetics of Fe-(18-20)Mn-(9-10)Al-(0.9-1.0)C steels with additions of 5 and 8%Ni was determined for aging temperatures between 450 and 600C. Adding Ni produced a high volume fraction of nano-sized NiAl precipitation within the austenite that greatly contributed to the strength in the as-annealed steel. Hardness increased with aging time as a result of kappa carbide precipitation within the austenite as well as NiAl ordering within ferrite stringers.

8:20 AM  
Microstructural Refinement in Martensitic and Maraging Steels: Vikas Sinha1; E.J. Payton2; M. Gonzales2; 1Air Force Research Laboratory/UES Inc. ; 2Air Force Research Laboratory
    Rapid austenitizing heat treatments were employed to refine microstructures of two martensitic steels (AF9628 and Secure 500) and one maraging steel (Ultrafort 6355). The phase transformation temperatures, A1 and A3, for different steels were estimated with thermodynamic modeling using PANDATTM and validated with experimental measurements using differential thermal analysis. Rapid austenitizing schedules consisted of rapid heating to temperatures slightly above A3, holding for short time, and quenching. The temperature and number of rapid austenitizing heat treatment cycles were varied to examine their effects on the level of microstructural refinement. The mechanisms of microstructural refinement were elucidated via characterizations with SEM and EBSD. The effects of microstructural refinement on microhardness were also assessed. The effects of microstructural refinement on dynamic/high strain-rate behavior are being investigated. (The authors thank Dr. R.A. Abrahams (AFLCMC) for providing AF9628 steel and Prof. W. Riedel (Fraunhofer-EMI) for providing Secure 500 and Ultrafort 6355 steels).

8:40 AM  
Mechanism for Z-phase Formation in 11CrMoVNbN Martensitic Heat-resistant Steel : Myung-Yeon Kim1; Jae-Hyeok Shim1; Young-Kook Lee2; Woo-Sang Jung1; Young-Su Lee1; 1Korea Institute of Science and Technology; 2Yonsei university
    Various 9–12 wt.% Cr martensitic heat-resistant steels have been developed and are commonly used for coal-fired power plants due to their high creep rupture strength and good oxidation resistance at high temperatures. The formation of coarse Z-phase precipitates with the tetragonal structure has been reported to reduce creep rupture strength after being exposed to high temperature. In this study, the mechanism for the formation of Z-phase was investigated for samples of an 11CrMoVNbN steel with high nitrogen content aged at 593 C for up to 50,000 h. It is confirmed that formation of V-rim and nucleation of Z-phase are caused by diffusion of V in a fine Cr2N precipitate after long-term aging at 593 C.

9:00 AM  
Effects of Heat Treatments on High Strength Low-alloy Nitrogen Steel: John Chinella1; 1U S Army Research Laboratory
    High-strength steels and ultra-high strength steels are susceptible to localization of flow and fracture during extreme loading and ballistic impact. The ensuing failure modes of localized plastic flow and fracture lower the levels of protection performance. Furthermore, sensitivity to stress corrosion cracking is of concern. As a possible approach for improvement, nitrogen alloying enables profound effects on atomistic bonding and offers novel thermodynamic, material, microstructural, and mechanical properties versus carbon steels. Nevertheless, the material properties and performance may be limited by the technical barriers of composition, processing, microstructure, material defects and impurities, the severity of service, and costs. This presentation describes developmental efforts in low alloy N-steel through responses of microstructures and properties following heat treatments. The material property objectives are high levels of strength and toughness and novel atomistic and microstructural properties which enable service with low-costs, improved protection, durability, and vehicle light-weighting.

9:20 AM  
Correlation of Microstructure and Hardness of Spheroidization Annealed Carbon Steel: Chang-Young Son1; 1POSCO/Hot Rolled & Wire Rod Research Group
    Spheroidizing is a form of heat treatment for carbon steels, in order to convert them into ductile and high formable steels. Hardness depending on tempering parameter of annealed steels has been investigated for many decades. However, the number of studies on microstructure of annealed steels is fewer than those of hardness. In this study, it was investigated correlation of microstructure and hardness of spheroidization annealed carbon steel. Hot rolled and cold rolled carbon steels were annealed, grain and carbide size of spheroidized cementite was measured with an image analyzer.

9:40 AM  
Effect of Zr Microalloying on Austenite Grain Size of Low-carbon Steels: Minghao Shi1; Rangasayee Kannan1; Lulu Guo1; Xiaoguang Yuan2; Leijun Li1; 1University of Alberta; 2Shenyang University of Technology
    Effect of Zr addition on characteristics of inclusions and the prior austenite grain sizes following a heat treatment has been investigated for two custom-made steels. Results show that ZrO2 and MnS exhibit a coherency relationship, significantly reducing the interfacial energy, promoting MnS precipitation on ZrO2. Although the size of inclusions in Zr-containing steel is the same as Zr-free steel (0.49μm), the total number of particles in Zr-containing steel is higher than Zr-free steel. The total number of particles in steel is related to the gravity density of particles. The density of TiO2 and ZrO2 are 3.4 g / cm3, 5.85 g / cm3, respectively, which is closer to that of liquid Fe (7.8 g / cm3). The prior austenite grain size for specimen of Zr-containing steel was always smaller than specimen of Zr-free steel due to many fine inclusions working as pinning particles to retard the growth of austenite grains.

10:00 AM Break

10:20 AM  Cancelled
Nanocrystallization during Plasma Nitriding of Steels: Jiawei Yao1; Mufu Yan1; Fuyao Yan1; 1Harbin Institute of Technology
    Nitriding is an effective thermochemical treatment process to improve the surface tribological and corrosion properties of steels via the presence of nitrides (ε-Fe2-3N and γ-Fe4N) in the surface layer. Instead of the formation of the compound layer, this work presents the observation of surface nanocrystallization during plasma nitriding of various types of quenched steels (e.g. 17-4 PH and M50) under specific nitriding conditions. Characterized by x-ray diffraction and transmission electron microscopy, nano-scale N-rich expanded martensite and N-lean FeN0.076 phase, associated with peak broadening, are evident. The nitrided layer exhibits ultra-high wear resistance. With the aid of thermodynamic modeling using the CALPHAD method, the Gibbs free energy of the nitrided steel system is calculated and its stability vs. N concentration is evaluated. It is found that nanocrystallization occurs when the Gibbs free energy of the nitrided system exhibits ‘double-well’, and when temperature and N concentration are within the spinodal.

10:40 AM  Cancelled
Effect of Plasma Nitriding at Different Temperatures on Microstructure and Mechanical Properties of AerMet100 Steel: Baofeng Chen1; Fuyao Yan1; Mufu Yan1; Yanxiang Zhang1; 1Harbin Institute of Technology
    As the service conditions of aircrafts becomes harsh, a hard and protective surface layer is always needed to improve the surface corrosion and tribological properties. Plasma nitriding is an economic and effective thermochemical treatment process to modify the surface. Moreover, plasma nitriding can be combined with high-temperature tempering due to the overlapping temperature window. In this work, AerMet100 landing gear steel is employed to evaluate the influences of nitriding temperatures on surface modification and secondary hardening effects. Quenched AerMet100 steel are plasma nitrided at temperatures between 430 and 550 oC, and the corresponding microstructures and mechanical properties are investigated. Results show that nitriding at intermediate temperatures has a nitrided layer free of brittle ε-Fe2-3N, along with finely dispersed M2X precipitates and reversed austenite, exhibiting a good combination of hardness, ductility and wear resistance. Too low temperature promotes the formation of brittle ε-Fe2-3N, while too high temperature leads to tempering softening.

11:00 AM  
Microstructural Evolution during Aging of a TRIP-aided Mo-free Lean Duplex Stainless Steel : Kyung-Tae Park1; Jeomyong Choi2; 1Hanbat National University; 2Zhangjiagang Pohang STS Co. Ltd.
    Precipitation and phase transformation during aging of a newly developed TRIP-aided Mo-free lean duplex stainless steel (DSS) was investigated. This DSS contains the reduced Ni and the increased Mn and N contents, compared to conventional DSS. In this study, Mo was intentionally removed to suppress sigma phase precipitation. Aging was conducted at temperatures of 600 ~ 1000 C up to 2 hrs. Under these aging conditions, Cr2N first precipitated at phase/grain boundaries as similar to the Mo-bearing DSS. But the sigma phase did not precipitate due to Mo-free chemistry. Instead, significant martensitic transformation occurred at the gamma austenite side, contrary to the Mo-bearing DSS in which precipitation and phase transformation occur mainly at the delta ferrite side. This behaviour was rationalized on the basis of elemental redistribution along with Cr2N precipitation during aging.

11:20 AM  
Effects of Microstructural Evolution on Corrosion Characteristics of Co-reduced UHS Stainless Steel: Sungsoo Park1; 1POSCO
    The effects of secondary hardening on the mechanical properties of ultra-high-strength stainless steels were investigated. Based on the chemical composition of Ferrium S53 (0.21C-10Cr-2Mo-1W-0.3V-14Co-5.5Ni-0.02Ti), a 7Co1V, 4Co1V alloys were designed by reducing the Co content from 14 to 7.0, 4.0 wt.% and slightly increasing the V and C contents from 0.3 to 1.0 wt.% and 0.21 to 0.25 wt.%, respectively. A multi-cryogenic tempering process was applied to reduce the retained austenite to the designed material. The desiged alloy modification changes the M2C-strengthened microstructure to an MC plus M2C-strengthened microstructure; thus, the Co-reduced and secondary hardened ultra-high-strength martensitic steel demonstrated enhanced corrosion resistance without any distinct reduction in its mechanical properties. As these results are closely related to the retardation of Cr-rich M23C6 by the change in precipitation kinetics associated with aging, corrosion characteristics under constant current was determined via MatCalc simulation and an investigation of its oxidation behavior upon heating.