材料期刊网

用金相方法研究了含少量钼、钛18/8 Cr-Ni不锈钢中δ-铁素体在900°—550℃恒温分解的过程。观察到在分解产物中没有σ-相。δ-铁素体分解形态及其机构随分解温度而有所不同。当分解温度高于750℃时,先析出奥氏体γ′,继之在未转变的铁素体中,才沉淀出碳化物。分解温度低于650℃时,则先析出碳化物,随后才出现有奥氏体γ′。介乎750°—650℃之间,δ-铁素体通过共析转变方式分解为碳化物及γ′。共析分解未能进行到底,在残留的铁素体中,有奥氏体γ′析出,由于合金元素分配的关系,δ→γ′的转变不久亦停止。 在试验过程中,观察到电解磨光后,在碳化物未全部溶解或δ-铁素体已发生分解的样品表面上,出现有马氏体。用同佯手续制备1300℃固溶处理(碳化物全部溶解)的样品,则没有出现马氏体。初步认为马氏体的出现是由于电解磨光过程中产生自由表面所引起的。但是这种表面马氏体的形成似亦与奥氏体的含碳量有关,其形成机构尚待进一步的研究。

The decomposition of δ-ferrite between 550°—900℃ in quenched 18/8 stainlesssteel specimens containing 0.34% Ti and 0.24% Mo has been studied microsco-pically. The mechanism of decomposition of δ-ferrite was found to be dependenton the reheating temperature. No sigma phase was found on prolonged heating inthe temperature range studied. Above 750℃, accicular austenite forms first, followedby general precipitation of TiC in the untransformed ferrite. Below 650℃, the se-quence of precipitation in ferrite is reversed, carbide precipitation precedes the for-mation of austenite. Between 650°—750℃, δ-ferrite decomposes into carbide andaustenite in an eutectoid manner. The eutectoid decomposition does not carry to itscompletion; part of the remaining ferrite then transforms into austenite which soonalso stops on account of the enrichment of δ-forming elements in the remainingferrite. Upheaved martensite plates were found in the electrolytically polished surface ofspecimens containing undissolved carbide and also in specimens in which the decompo-sition of ferrite has started when prepared in the same way. However, when allcarbide is dissolved, no martensite is found in the surface of the specimens whetherit is prepared electrolytically or mechanically. It is believed that the formation ofmartensite in this particular steel is a surface effect associated with electrolytic poli-shing, but its formation is also closely related to the carbon content of the auste-nitic matrix.