Hydrogen Effects on Austenitic Stainless Steels and High-Strength Carbon Steels

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Olga Madelen Ingrid Todoshchenko Name of the doctoral dissertation Hydrogen Effects on Austenitic Stainless Steels and High-Strength Carbon Steels Publisher School of Engineering Unit Deparment of Engineering Design and Production Series Aalto University publication series DOCTORAL DISSERTATIONS 68/2015 Field of research Engineering Materials Manuscript submitted 9 February 2015 Date of the defence 12 June 2015 Permission to publish granted (date) 20 April 2015 Language English Monograph Article dissertation (summary + original articles) Abstract The resistance to hydrogen embrittlement is an important factor in the development of new steel grades for a variety of applications. The thesis describes investigations on hydrogen effects on two classes of steels austenitic stainless steels and advanced high-strength carbon steels. Hydrogen solubility and diffusion in metastable austenitic stainless steels are studied with thermal desorption spectroscopy (TDS). This method, together with the mathematical modeling of the processes of hydrogen uptake and desorption, allows to analyse hydrogen diffusion and trapping in a metal. Temperature dependencies of hydrogen desorption for the studied steels manifest a complex peak. Specific features of hydrogen uptake and desorption for a multi-component alloy in comparison with that for pure metals are analysed by the proposed model. It was found that plastic strain affects the shape of the TDS peak for all the studied materials. In metastable austenitic stainless steels the parameters of hydrogen diffusion and trapping in martensite phase, which is forming under pre-straining, were estimated by the proposed model. High-strength carbon steels of the strength level from 1000 to 1400 MPa with different contents of Ti were studied. It was found that there is an optimal content and size distribution of Ti-based non-metallic inclusions (NMI) when the steel is most resistant to hydrogen embrittlement. Fractography of the studied steels shows that the fracture mechanism depends on the chemical composition of the studied steels and hydrogen-induced cracking exhibits intergranular or transgranular character occurring often in the form of hydrogen flakes. It was found that hydrogen-induced cracks of the hydrogen flakes initiate at Ti-based NMIs. TDS analysis evidences that the main trapping sites for hydrogen in the high-strength carbon steels are NMI interfaces. The mechanisms of hydrogen interaction with NMIs are discussed. All the studied high-strength carbon steels are sensitive to hydrogen under slow strain rate tensile tests. Constant load tests show that different processes influence on hydrogen-induced fracture at low and high applied stresses for the high-strength carbon steels with high Tialloying. Tempered high-strength carbon steels are less sensitive to hydrogen embrittlement than their non-tempered counterparts.The resistance to hydrogen embrittlement is an important factor in the development of new steel grades for a variety of applications. The thesis describes investigations on hydrogen effects on two classes of steels austenitic stainless steels and advanced high-strength carbon steels. Hydrogen solubility and diffusion in metastable austenitic stainless steels are studied with thermal desorption spectroscopy (TDS). This method, together with the mathematical modeling of the processes of hydrogen uptake and desorption, allows to analyse hydrogen diffusion and trapping in a metal. Temperature dependencies of hydrogen desorption for the studied steels manifest a complex peak. Specific features of hydrogen uptake and desorption for a multi-component alloy in comparison with that for pure metals are analysed by the proposed model. It was found that plastic strain affects the shape of the TDS peak for all the studied materials. In metastable austenitic stainless steels the parameters of hydrogen diffusion and trapping in martensite phase, which is forming under pre-straining, were estimated by the proposed model. High-strength carbon steels of the strength level from 1000 to 1400 MPa with different contents of Ti were studied. It was found that there is an optimal content and size distribution of Ti-based non-metallic inclusions (NMI) when the steel is most resistant to hydrogen embrittlement. Fractography of the studied steels shows that the fracture mechanism depends on the chemical composition of the studied steels and hydrogen-induced cracking exhibits intergranular or transgranular character occurring often in the form of hydrogen flakes. It was found that hydrogen-induced cracks of the hydrogen flakes initiate at Ti-based NMIs. TDS analysis evidences that the main trapping sites for hydrogen in the high-strength carbon steels are NMI interfaces. The mechanisms of hydrogen interaction with NMIs are discussed. All the studied high-strength carbon steels are sensitive to hydrogen under slow strain rate tensile tests. Constant load tests show that different processes influence on hydrogen-induced fracture at low and high applied stresses for the high-strength carbon steels with high Tialloying. Tempered high-strength carbon steels are less sensitive to hydrogen embrittlement than their non-tempered counterparts.