Nobuo Nakada

Influence of Chromium on the Hall-Petch Coefficient in Ferritic Steel

In ferritic stainless steels, the amount of Cr is moderately controlled to have good corrosion resistance in applied environment. However, it also affects the yield strength of ferritic stainless steels through solid solution strengthening and grain refinement strengthening. Until now, some researches have been performed using commercial stainless steels but the obtained results contain the effect of solute interstitials (C and N). In this paper, the influence of Cr on the above both strengthening mechanism was discussed by using interstitial free ferritic stainless steel in which carbon and nitrogen are completely fixed as Ti(C,N). A previous paper has reported that the addition of chromium gives different influences to the Hall-Petch coefficient depending on the amount of Cr. However, our research has reveals the fact that the change of Hall-Petch coefficient is not due to the effect of chromium but due to small amount of carbon which exists as an impurity in ferritic stainless steels. It was concluded that chromium itself does not give any influence to the Hall-Petch coefficient of ferritic iron.

Temperature Dependence of Austenite Nucleation Site on Reversion of Lath Martensite

The temperature dependence of austenite nucleation behavior within lath martensitic structure was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel partially reversed at (austenite + ferrite) two phase region. The shape and nucleation site of the reversed austenite grains were varied depending on the reversion temperature; fine acicular austenite grains frequently formed along the lath boundaries at a temperature lower than 915 K, while the granular ones tended to nucleate mainly on the prior austenite grain boundaries at a higher temperature. In order to explain the temperature dependence of nucleation site transition, the difference in energetics of austenite nucleation between the lath boundary and the prior austenite grain boundary was discussed on the basis of the classical nucleation theory and FEM analysis. The calculation of the changes in interfacial energy and elastic strain for austenite nucleation suggested that the lath boundary acts as more preferential nucleation sites for austenite rather than the prior austenite grain boundary to reduce the increment of elastic strain when the reversion temperature is low.

Multiple Precipitation Behavior of Niobium Carbide and Copper in Martensitic Steel

The multiple precipitation behavior of NbC and Cu particles in martensitic structure was investigated by using 0.05C-0.46Nb-2Cu-1.5Mn steel (NbC-Cu steel). Additionally, 0.05C-0.45Nb-2Mn steel (NbC steel) and 2Cu-5Mn steel (Cu steel) were also prepared to examine the respective precipitation behaviors of NbC and Cu. Aging treatment at 873K after quenching revealed that these steels exhibit typical age hardening. Comparing the NbC steel and Cu steel in the precipitation rate, the Cu precipitated much faster than the NbC. On the other hand, the peak hardness in NbC-Cu steel is higher than that by the respective precipitations in NbC steel and Cu steel. Besides, the aging time for the peak hardness in NbC-Cu steel was between those in NbC steel and Cu steel. This suggests that the NbC and Cu particles were separately precipitated within martensite matrix and each of them contributed to the hardening in NbC-Cu steel. As a result of TEM investigation for crystallographic characteristics of the precipitates, the NbC and Cu particles had different crystallographic orientation relationship with tempered martensite matrix: Baker-Nutting relationship for NbC particle and Kurdjumov-Sachs relationship for Cu particle.

Proposal for an engineering definition of a fatigue crack initiation unit for evaluating the fatigue limit on the basis of crystallographic analysis of pearlitic steel

In this study, in order to define a fatigue crack initiation unit, the relationship between the fatigue crack initiation process and the crystal structure in the pearlitic steel used for railroad rails was examined and fatigue tests, focusing on crack initiation, were performed. The fracture surfaces were analyzed using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The crystal structure of the pearlitic steel is composed of “pearlitic colonies” that have the same lamellar structure direction and “pearlitic blocks” that have the same ferrite crystal direction. The SEM and EBSD results revealed that the crack initiation depends on the pearlitic colonies. Therefore, we defined the characteristic dimension for fatigue crack initiation as the pearlitic colony size. However, for safety purposes, the pearlitic block size should be considered the engineering definition of the fatigue crack initiation unit, since decreasing the pearlitic block size should cause an improvement in the fatigue limit of pearlitic steel.

Decomposition of Austenite in Fe-25Cr-1N Alloy Produced by Solution Nitriding

The nickel-free austenitic stainless steel produced by solution nitriding (Fe-25%Cr-1%N alloy) was subjected to isothermal heat treatment, and then the microstructure formed through the decomposition of austenite was investigated in terms of the morphology of eutectoid structure and the size of eutectoid block. On the isothermal heat treatment at 873K~1223K for the solution-nitrided steel, the austenite decomposed to eutectoid structure composed of ferrite and Cr2N nitride. This transformation could be completely finished after long time heat treatment in the above temperature range. The nose temperature of T.T.T. curve was around 1173K, and the time to start the eutectoid transformation was only 100~200s. The eutectoid structure was formed mainly along austenite grain boundaries and then grew into the untransformed austenite region. Finally, the austenite was completely decomposed into ferrite and Cr2N nitride. As a result of OIM observation for the specimen after isothermal heat treatment, the eutectoid structure was found to be divided into small-sized ferrite blocks, in which lamellar Cr2N plates were finely distributed. The block size and the mean ferrite path of eutectoid structure were decreased with lowering the heat treatment temperature. In the 873K heat-treated material, these values were estimated at 20 microns and 0.1 microns, respectively.

Effect of Cooling Rate after High Temperature Nitriding on Transformation Microstructure in Low Carbon Steel

High temperature nitriding was applied to a low carbon steel, and then the transformation microstructure formed from the high nitrogen austenite was observed. The microstructure was greatly influenced by cooling rate after the nitriding. The austenite transformed to martensite when the cooling rate was fast (water-cooling) though a certain amount of austenite was retained in the martensite. On the other hand, a diffusional transformation product such as (ferrite + Fe4N) eutectoid structure was formed when the cooling rate was slow (gas- or furnace-cooling). The hardness profile obtained in these specimen were related with microstructure depending on the nitrogen concentration profile and cooling rate.

Microstructural Control of Dual Phase Structure Formed by Partial Reversion from Cold-Deformed Martensite

Dual phase (DP) structure formed by partial reversion from cold-deformed martensite was investigated to improve mechanical property of DP steel by grain refinement strengthening. A low carbon martensitic steel (0.15C-1.0Mn) was cold-rolled and then held just above A1 temperature to partially form austenite. In particular, the conditions of cold-rolling rate (0~60% reduction in thickness) and heating rate (0.083 and 100 K/s) were varied to understand their effects on the microstructural development of DP structure. Although the recrystallization has never occurred in undeformed martensite, cold-deformed martensite was more easily recrystallized before reversion with increasing rolling rate and lowering heating rate. Then, the matrix of DP structure was changed from tempered martensite to recrystallized ferrite, which had a large influence on the distribution of fresh martensite transformed from reversed austenite. The higher rolling and heating rates resulted in the finer DP structure, leading to a large improvement in strength level.

Microstructure Control of a Low Carbon Martensitic Stainless Steel by Quenching and Partitioning Heat Treatment

Quenching and partitioning (Q&P) treatment was applied to a commercial low carbon martensitic stainless steel, AISI Type 410 (Fe-12Cr-0.1C). The condition of partial quenching and partitioning was optimized with consideration of the untransformed austenite fraction and stability of austenite (carbon concentration in solid solution). As a result, the amount of retained austenite could be increased up to approximately 15 vol%. Tensile testing revealed that the specimens after Q&P heat treatment exhibited lower yield stress and larger work hardening rate compared with quench-and-tempered (Q&T) specimens under the same tensile strength level, resulting in a significantly better strength-ductility balance. It was confirmed that the TRIP effect had contributed to the mechanical property.

Microstructural and Crystallographic Characteristics of Deformation-Induced Martensite Formed in Cold-Drawn 316 Type Stainless Steel

Deformation-induced martensite preferentially nucleates at the twin boundary between matrix austenite and deformation twin in 316 type stainless steel. In the cold-rolled specimen, the martensite formed at the twin boundary has K-S relationships with both of the austenite matrix and the deformation twin, that is, “double K-S relationship” is realized. While in the case of cold-drawn specimen, two kinds of twins with different twin planes are typically observed, and therefore, the deformation-induced martensites are formed at the intersections of the two deformation twin boundaries, satisfying “triple K-S relationship” among austenite matrix and two deformation twins, although there is a small misfit from the perfect K-S relationship. The complicated crystallographic orientation relationship leads to a strong variant restriction for deformation-induced martensites. Due to the difference in the number of nucleation sites, martensitic transformation is greatly promoted in cold-drawn specimen rather than cold-rolled one.

Grain Boundary Carbon Segregation Estimated by McLean and Seah-Hondros Models

Our recent study has revealed that the Hall–Petch coefficient in ferritic iron has clear carbon concentration dependence in the range up to 60 ppm-C and it has been proposed that such a behavior deeply relates to the segregation of carbon at grain boundary. On the estimation of carbon concentration at grain boundary, two kinds of model could be applied: McLean model and Seah-Hondros model. In this study, the carbon concentration at grain boundary in ferritic iron was estimated by using these models for the temperature of 973 K. Under the presumption that the saturation atomic carbon concentration at grain boundary is 0.25 (equivalent to Fe3C), 6 ppm-C and 57 ppm-C were obtained from McLean model and Seah-Hondros model, respectively, for the critical matrix concentration that causes the grain boundary saturation. Hence, it was concluded that Seah-Hendros model is reasonable to explain the carbon concentration dependence of the Hall–Petch coefficient in ferritic iron.