AISI D2 강의 미세조직 및 탄화물 석출에 미치는 열처리의 영향

Abstract

The effect of heat treatment on the transformation characteristics and mechanical property has been studied by metallography, X-ray diffractometry, image analysis and hardness measurement in AISI D2 tool steel for cold working. The results obtained are as follows;

(1) As austenitizing temperature increases, the volume fraction of retained austenite(Rγ) increases. The increase in the volume fraction of Rγ is due to the falling of the Ms temperature resulting from the chemical stabilization of parent phase.

(2) As austenitizing temperature increases, Vickers hardness value decreases. The change in hardness value with austenitizing temperature is also affected by grain size as well the volume fractions of Rγ and carbides.

(3) The volume fraction of Rγ in the quenched AISI D2 tool steel, which consists of lots of carbides, austenite and α'-martensite, can be quantitatively calculated by using CuKα characteristics X-ray and image analysis. However, the values calculated from X-ray diffractometry do not accord with the values obtained by image analysis, and both values need to revise by introducing the compensation coefficient.

(4) The volume fraction of Rγ decreases by the sub-zero treatment but some untransformed austenite still remains even if immersing at -196 ℃. The existence of untransformed austenite is caused by the decrease in Ms ∼ Mf temperature below -196 ℃ due to the chemical stabilization of parent phase.

(5) The volume fractions of Rγ and carbides, tetragonality of martensite(c/a) and hardness are changed as a result of cryogenic treatment at -196 ℃ for 24hrs after quenching. Those values are closely correlated to the temperature of austenitization carried out before cryogenic treatment.

(6) When specimens are tempered, which are cryogenic-treated after quenching, at 200 ∼ 400 ℃ for 30 min, the M7C3 carbides are precipitated through rod-shaped MC or M2C transition carbides in the size of about 80 nm in matrix. The Vickers hardness value decreases with increase in tempering temperature.