Influence of Chemical Inhomogeneities on Local Phase Stabilities and Material Properties in Cast Martensitic Stainless Steel

Abstract

Cr-Mo-alloyed cast martensitic stainless steels are suitable tool materials for a wide field of applications. Local inhomogeneities in the chemical composition, however, affect their local and global properties such as the hardenability and the corrosion resistance. Herein, the influence of microsegregations on phase stabilities and properties is investigated by means of property distribution maps (PDM) which are determined via thermodynamic and empirical calculations based on measured local chemical composition data. The results show that the enrichment of Cr and Mo in interdendritic regions benefits the local corrosion resistance but increases the solvus temperature of M 23 C 6 carbides from 1040 to 1150 C and depresses the martensite start temperature (M s ) to temperatures below 50 C locally. As predicted from the PDM, high-temperature austenitization at 1150 C combined with a cryogenic treatment at À80 C ensures a martensitic microstructure with relatively high hardness (592 AE 12 HV10) and significantly higher critical pitting potential compared with specimens austenitizized at 1050 C, which proves PDM to be a powerful tool for the optimization of heat treatment parameters. However, local transformation of austenite into δ-ferrite during austenitization at 1150 C must be considered.

Effect of the Microsegregation on Martensitic and Bainitic Reactions in a High Carbon-High Silicon Cast Steel

“…According to the seminal work of Andrews [67] and many others that have followed afterwards [68][69][70][71] different local Ms temperatures could be expected as a result of the microsegregation and this could also affect the kinetics of this transformation. In steels having an inhomogeneous distribution of alloying elements in the matrix, a splitting in the Ms or a gradual instead of sudden change in the slope below the Ms has been observed [63][64][65]72]. It is also well-known that the austenite grain size affects the stability of the austenite and its transformation to martensite as discussed by several authors [37,73,74] and this could also affect the onset and progress of the transformation.…”

“…Solute (interdendritic) rich zones, with higher Si, Mn, and Cr concentration, would possess a smaller ΔT (less thermal energy to grow), promote solute drag effect and contain a greater amount of submicrometer size cementite undissolved precipitates, that would pin the grain boundaries, factors contributing to having a smaller austenite grain size. As both the composition and austenite grain size may influence the bainitic and martensitic transformations [26][27][28][34][35][36][63][64][65], it should be expected that the microsegregation and the inhomogeneous grain size distribution will affect these transformations. Microstructures have been etched with Vilella reagent.…”

Influence of nitrogen uptake and heat treatment on the microstructural characteristics and corrosion performance of X190CrVMo20‐4‐1 steel produced by supersolidus liquid‐phase sintering

“…[17] The empirical formula for the martensite start temperature was adjusted by considering the nitrogen to exhibit similar influence on the matrix austenite stabilisation and hardenability as carbon. [7] PREN = Cr + 3.3Mo + 20N (all in wt%),…”

“…As Cr and Mo possess much lower diffusivities when compared with that of the interstitial elements (C and N), the microsegregated Cr and Mo will not fully homogenise during the duration of austenitisation. [7] The increasing content of these elements lower the martensite start temperature, such that the enriched regions are austenite-stabilised after quenching to room temperature. [22] As a low-temperature tempering at 200°C was adopted to relieve the internal stress in the martensite, the retained austenite colonies remained undecomposed or transformed, as seen in Figure 4a-c,e,f.…”

“…The solubility of nitrogen is always higher in the austenite phase of iron and nitrogen has a similar effect as carbon on stabilising the steel as an austenitic phase. [7] In addition, the nitrogen solubility in the austenite iron (γ-Fe) is reported to be dependent on temperature, pressure and alloy composition. As for tool steel, it is desired that the matrix becomes martensitic upon rapid quenching from a suitable austenitising temperature.…”