Process for the treatment of an optimised steel material

A process for treating a steel material, comprising:

• hot isostatic pressing the steel material,
• wherein the steel material is obtained by an additive manufacturing process chosen among a selective laser melting process, a selective electron beam process, and a laser spraying process,
• wherein the steel material comprises grains comprising a matrix into which precipitates are incorporated,
• wherein
• (i) the steel material comprises, as a percentage by weight:
• chromium in a range of from 17.26% to 20%;
• nickel in a range of from 8% to 14%;
• carbon in a range of from 0.001% to 0.030%;
• oxygen in a range of from 0.001% to 0.050%;
• manganese in a range of from 0% to 2%;
• molybdenum in a range of from 0% to 3%;
• silicon in a range of from 0% to 1%;
• nitrogen in a range of from 0% to 0.11%;
• phosphorus in a range of from 0% to 0.045%;
• sulfur in a range of from 0% to 0.05%;
• aluminum in a range of from 0% to 0.0300%;
• vanadium in a range of from 0% to 0.003%;
• cobalt in a range of from 0% to 0.02%;
• titanium in a range of from 0% to 0.02%;
• copper in a range of from 0% to 0.02%;
• and
• iron, and
• (ii) the precipitates comprise at least one metallic element selected from the group consisting of a metallic element M, a metallic element M′, and a metallic element M″, each of the metallic elements M, M′ and M″ being, if present, at least one selected independently from the group consisting of yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminum, hafnium, and molybdenum,
• wherein the steel material has, before and after the hot isostatic pressing, a microstructure of equiaxed grains having an average grain size average size in a range of from 10 μm to 50 μm at a largest dimension Dmax and/or a smallest dimension Dmin,
• wherein, before the hot isostatic pressing, the steel material has a microstructure comprising grains having a cellular structure, cells of the cellular structure having an average size in a range of from 300 nm to 2 μm,
• wherein the precipitates are distributed in the matrix in an average density in a range of from 2 to 100 precipitates/μm3,
• wherein the hot isostatic pressing comprises, in succession, carried out in a chamber comprising an inert gaseous atmosphere under a pressure in a range of from 120 bar to 1800 bar:
• (a) heating the steel material to a constant temperature in a range of from 600° C. to 1400° C. at a temperature ramp-up rate in a range of from 500° C./hour to 1000° C./hour;
• (b) maintaining a constant temperature for a period in a range of from 15 minutes to 5 hours;
• and
• (c) reducing the constant temperature at a temperature ramp-down rate in a range of from 500° C./hour to 1000° C./hour in order to reach ambient temperature,
• wherein the grains of the steel material are equiaxed in a plane which is parallel to a plane of superimposed layers of a material manufactured by the additive manufacturing process, and
• wherein the grains of the steel material are equiaxed in a plane which is perpendicular to a plane of superimposed layers of a material manufactured by the additive manufacturing process.