Il trattamento di solubilizzazione degli acciai inossidabili duplex

摘要

It is well established that duplex stainless steels have a favorable combination of mechanical properties and corrosion resistance. Such good properties are related to the two-phase microstritcture and to the control of secondary phases precipitation. The solution treatment optimization is therefore necessary to avoid secondary phases precipitation and to keep a/g phase balance. In the last few years' new duplex stainless steels with different compositions were produced especially to improve corrosion resistance. In this paper the solution treatment conditions of three commercial duplex stainless steels (1.4462, 1.4460 and 1.4410) were examined in the temperature range of 1050-1100 deg C. Solution heat treatments, necessary to restore the a/g phase balance and solubilize unwanted phases, have to be sufficiently rapid in order to avoid their precipitation. Precipitation times of c and s phase were determined in the temperature range of 750-950 deg C with isothermal treatments. Moreover continuous cooling treatments were used to determine minimum cooling rates to avoid-secondary phases formation. The lowest solubilization temperature (1050 deg C) is determined by the request to solubilize secondary phases precipitated after prior heat treatments while the higher solubilization temperature (1100 deg C) is determined by the request to keep a/g phase balance. The precipitation kinetics of secondary phases in the critical temperature range strongly determines the minimum requested cooling rate. For duplex stainless steels with low chromium content the critical cooling rate is about 0,3 deg C/s, while for higher chromium and molybdenum contents is about 0,l deg C/s, which is not easily achievable at the core of pieces with a diameter greater than 80 mm.%Sono state esaminate le condizioni per il trattamento termico finale di alcuni fra gli acciai duplex di piu largo impiego. Dopo il mantenimento a temperature comprese fra 1050 e 1100 deg C, necessario per ripristinare il corretto rapporto fra austenite e ferrite e solubilizzare le eventuali fasi dannose, deve essere eseguito un raffreddamento sufficientemente rapido, per evitare che queste possano riprecipitare. I tempi di precipitazione delle fasi X e sigma nell'intervallo critico di 750-950 deg C sono stati determinati con trattamenti isotermici e raffreddamenti a velocita controllata, che hanno consentito di determinare le velocita di raffreddamento minime necessarie per evitare la formazione di queste fasi.