Effect Of Heat Treatment On The Structure And Mechanical Properties Of Superaustenitic Stainless Steel_eti [efeito Do Tratamento Térmico Na Estrutura E Nas Propriedades Mecânicas De Um Aço Inoxidável Superaustenítico]

Abstract

The austenitic stainless steel high alloyed (22Cr-25Ni-7Mo-0,3N), well known as superaustenitic, has as its main characteristic a high corrosion resistence to several environments, mainly in sea water. Furthermore the material shows good mechanical properties and thermal stability. To obtain the mentioned characteristics, the material must be thermically treated, taking into consideration the solubility of the alloy elements. Various heat treatments were performed in this research : solution heat treatment (from 1100 to 1250°C), stress relief (from 500 to 800°C) and sensitization (900°C, varing the time at 1,5h, 12h, 24h and 48h). Hardness and charpy tests were used to study the mechanical behaviour, and optical methalography was done to investigate the heat treatment influence in the material micro structure. It was observed that even with the solution at an extreamely high heat treatment temperature (1250°C), it was not possible to dissolve the precipitates formed during solidification scattering in the austenitic matrix. To maximize the energy absortion impact, the solution heat treatment must be performed between 1150°C and 1200°C and the stress relief until 500°C. In the 900°C temperature range, the material impact resistence (measured at environment temperature) decreasedfrom 25J to 7J when the heat treatment time was increased from 1,5h to 48 h.601155161ASTM A890/A890M - 91. Standard practice for castings, iron-chromium-nickel-molybdenum corrosion-resistant, duplex (Austenitic/ ferritic) for general application. American Society for Testing and Materials. Easton. v. 01.02. Ferrous Castings, Ferroalloys, p.556-569GRUBB, J.F., DEEMER, D.E., AL-6XN Alloy (2002) Allegheny Ludlum, pp. 2-7. , USA, pCURTIS, J.F., KOVACH, W., (2002) High performance stainless steel, p. 17. , USA: Níquel Development InstitutePADILHA, A.F., RIOS, P.R., Decomposition of austenite in uustenitic stainless steel (2002) USP: Departamento de Engenharia Metalúrgica e de Materials, pp. 325-337/ A744M. Standard specification for casting, Iron-chromium, iron-chromium-nickel, corrosion resistant, for general application (1998), pp. 1-6. , ASTM A744, American Society for Testing and Materials, U.S.A(1998) Corrosion-resistant steel casting, p. 8. , EUROPEAN STANDARD EN 10283, DIN Deutsches Institute fur Nourmung, Germany, BerlinNORSOK STANDARD M-630 MDS R16. Material data sheets for piping. Standards Norway, Norway, 2004Standard Test Methods and Definition for Mechanical Testing of Steel Products (1997), pp. 7-10. , ASTM A370, American Society for Testing and Materials, U.S.AStandard hardeness convertion tables for metals (2002), p. 893. , ASTM E 140, American Society for Testing and Materials, U.S.