Shuji Hamano

Properties of Low Carbon High Nitrogen Martensitic Stainless Steels

We produced low carbon and high nitrogen martensitic stainless steels that contain less than 0.1 mass% C and more than 0.45 mass% N, through the pressurized induction melting process, in which nitrogen is introduced from a pressurized N2 atmosphere. The hardness and corrosion resistance of these steels were investigated under various heat treatment conditions. The hardness of these steels after spheroidal annealing treatment is approximately 95HRB and the cold workability is superior to that of AISI440C. The hardness of these steels after quenching and sub-zero treatment is from 53 to 56HRC. In the tempering process, however, high nitrogen steels show secondary hardening at approximately 4 points in HRC compared with the quenched hardness after subzero treatment and have the maximum tempered hardness of 56 to 60HRC around 723K. The corrosion resistance of quenched and tempered materials under 723K is better than AISI304 evaluated by the pitting potential in 3.5% NaCl aqueous solution. Both remnant Cr2N in hardening and precipitated Cr2N in tempering degraded the corrosion resistance of high nitrogen martensitic stainless steels. The best balanced developed steel has a hardness of 60HRC and better corrosion resistance than AISI304 under optimal heat treatment conditions.

Development of Thermal Fatigue Resistant Austenitic Cast Alloys for High Temperature Engine Exhaust Gas Systems

As trends of automobile engine exhaust gas temperature are reducing emissions, the material for the exhaust components have been changed from ductile irons to ferritic cast alloys or stainless steel, further to austeniticcast alloys for higher performance engines. The current austenitic alloys, however, have thermal fatigue failure over 1273K. The authors developed excellent thermal fatigue resistant austenitic cast alloys, by investigating the effects of alloying elements on strength and thermal expansion, which correlate with thermal fatigue property. Developed alloys are expected to apply to exhaust components at gas temperatures over 1273K.