Premium
Effect of solidification conditions on the solidification sequence of austenitic chromium‐nickel stainless steels
Author(s) -
Siegel Ulrich,
Spies HeinzJoachim,
Eckstein HansJoachim
Publication year - 1986
Publication title -
steel research
Language(s) - English
Resource type - Journals
eISSN - 1869-344X
pISSN - 0177-4832
DOI - 10.1002/srin.198600716
Subject(s) - austenite , materials science , metallurgy , ferrite (magnet) , ingot , crystallization , chromium , alloy , saturation (graph theory) , microstructure , thermodynamics , composite material , physics , mathematics , combinatorics
Unlike the well‐known effect of alloy elements in promoting the ferritic or austenitic solidification of stainless and acid‐resisting chromium‐nickel steels, kinetic effects have as yet not been so widely looked into. For this reason, the impact of the solidification rate on the ratio of the amounts of ferritic and austenitic liquid solidification was investigated for the steels of grades X8CrNiTi18.10 and X8CrNiMoTi18.11. A microanalysis for the determination of the primary ferrite content of samples taken from ingots of different size and at different distances from the ingot surface for a total of 161 heats revealed the following: – Increasing solidification rate causes the primary ferrite content produced during solidification to rise for steels with peritectic solidification sequence due to the resultant approach of the distribution coefficient to unity. – Increasing solidification rate causes the austenite content to rise for steels with a primary simultaneous crystallization of austenite and ferrite due to a low total segregation in case of austenite crystallization as compared with ferrite crystallization. – The effect of an elevated solidification rate is qualitatively equivalent to a shift of the saturation lines of the three‐phase space l+δ+γ in the Fe—Ni—Cr ternary system for liquid and γ‐crystals in the direction S with the two saturation lines approaching each other. Hence, contrary to what is expected according to the equilibrium diagram of Schürmann and Brauckmann, austenitic Cr—Ni steels solidify primarily in peritectic mode and, in the area of the line of the double‐saturated liquid, through a primary simultaneous crystallization of austenite and ferrite. – The boundary composition between primary ferritic and primary austenitic crystallization changes with an increase in cooling rate by seven orders of magnitude from 1.25 to 1.70 as expressed in the ratio of the Cr—Ni equivalents according to Hammar and Svensson.