| 18Ni300 maraging steel is an advanced ultra-high strength steel,which is widely used in aerospace,high-end molds and other fields.In recent years,because of the high efficiency,personalization,excellent performance and many other advantages,the maraging steel by laser selective melting(SLM)technology have attracted much attention.In the actual service condition,SLMed maraging steel also faces the risk of hydrogen embrittlement due to its high strength,its hydrogen embrittlement behavior is not fully revealled at present.Therefore,in this work,the differences of microstructure and hydrogen embrittlement behavior between SLM and conventional18Ni300 maraging steel were firstly compared,then the effects of aging treatment time(6,9,12h)and temperature(390,490,590 ℃)on the hydrogen diffusion and hydrogen embrittlement behavior of SLMed 18Ni300 maraging steel were studied using a combination of microstructure characterization,hardness test,tensile test,hydrogen penetration test,and fracture analysis.The main conclusions are as follows:Compared with conventional maraging steel,SLMed maraging steel has higher strength,better plasticity and better hydrogen embrittlement resistance.Its better hydrogen resistance is mainly due to its unique microstructure: first,a large number of submicron cell structures can capture hydrogen,lead to a more evenly distributed of hydrogen and hinder the enrichment of hydrogen at the defects,therefore,inhibit the initiation and propagation of hydrogen embrittlement cracks;On the other hand,the proportion of Σ3 boundary is lower,which also helps to inhibit the propagation of hydrogen induced crack and improve the resistance to hydrogen embrittlement.With the increase of aging time,the hydrogen embrittlement susceptibility of SLM 18Ni300 maraging steel first increases and then decreases,which is mainly related to the competition between the strength and precipitates during aging: on the one hand,the increase of aging time will enhance the strength,thus lead to a higher hydrogen embrittlement susceptibility of steel;On the other hand,the increase density of Ni3 X precipitate will provide more hydrogen trap sites,homogenize the distribution of hydrogen,slow down the hydrogen enrichment at the defects,and improve the hydrogen embrittlement resistance.Among them,after aging for 12 h,the beneficial effect of nano precipitates far exceeds the adverse effect of increasing strength,so that the steel shows excellent hydrogen embrittlement resistance.With the increase of aging temperature,the hydrogen diffusion coefficient of SLM 18Ni300 martensitic steel first decreases,then increases and then decreases,which is mainly due to the complex changes of precipitates,reverse austenite,cell structure and their coupling effect on hydrogen diffusion.After aging treatment at590 ℃,the steel has excellent hydrogen embrittlement resistance,which is mainly due to the dispersed Ni3 X nanoprecipitates in the matrix,the effect of a large amount of austenite in inhibiting the local hydrogen enrichment at the defects,and the enhanced crack propagation resistance by lower Σ3 boundary. |
