Effect of Heat-Treatment on 18-12 Stainless Steel Containing Zirconium

Study of 18-12 stainless steel containing zirconium-I

Abstract

Thirteen kinds of austenitic 18-12·stainless steels containing zirconium up to 2·81% were melted in vacuum. The behaviors of carbide, nitride and some other phases in the forged specimens of these steels during heat treatments were studied by examining the austenite grain size, hardness and microscopic structures.
At the same time, these phases were extracted from these specimens electrolytically and identified by the X-ray analyses or electron-diffraction method. Results obtained are as follows;
(1) Austenite grain size of 18-12 stainless steel is refined by the addition of a small amount of zirconium such as 0·02%.
(2) The microstructures of each ingot containing zirconium shows the precipitation of ZrC and the quantity of ZrC is increased as the content of zirconium is increased. An intermetallic compound Fe₂Zr is found in the steels containing zirconium higher than 1·15%. In addition to these phases, some zirconium nitrides of cubic structure are found in each steel.
(3) In the specimens solution-treated at 1100°C for 3h, M₂₃C₆ dissolves into the matrix, but, even. in steels containing a small amount of zirconium. a part of ZrC remains as the undissolved carbide.
Though the solution treatment at higher temperature such as 1300°C causes the austenite grain growth and decreases hardness, it is impossible to dissolve ZrC completely.
(4) Burning structures are found on the austenite grain boundaries. and the intermetallic compound Fe2Zr is dissolved by the solution treatment at 1300°C for 3h.
(5) The precipitation of M₂₃C₆ is observed in the specimens containing a small amount of zirconium by annealing at 600°C for 100h, but the quantity of this precipitant is decreased as the content of zirconium is increased, and this carbide is not observed microscopically in the specimen containing 0·87% zirconium.
(6) The precipitation of M₂₃C₆ is accelerated by annealing at 900°C and the carbide is already observed at this annealing temperature within 5h.
(7) M₂₃C₆ cannot be observed in the specimens annealed at 1000°C while ZrC seems to be precipitated at this temperature within 100h.
(8) This carbides M₂₃C₆ is identified to be (Fe·Cr) ₂₃C₆ from the results of electrondiffraction pattern.