PURPOSE: To obtain the titled insert with superior oxidation resistance and thermal fatigue resistance by using a heat resistant steel for forging having a martensite structure and a specified composition consisting of C, Si, Mn, Ni, Cr, Mo, W, V and Fe.

CONSTITUTION: This insert for a precombustion chamber is made of a heat resistant steel for forging having a martensite structure and a chemical composition consisting of 0.25W0.4% C, 1.5W2.5% Si, ≤1% Mn, 0.2W1% Ni, 12W14% Cr, 0.5W1.5% Mo, 0.15W1% W, 0.15W1% V and the balance Fe with impurities. The insert can be formed by forging. It is hardly cracked even when it undergoes repeated heating at ≤ about 800°C during use. The insert has high oxidation resistance and corrosion resistance besides said superior properties, so it is fit for long-period use.