Insights into Primary Carbides and Nanoparticles in an Additively Manufactured High-alloy Steel
This paper reports the use of combined microscopy and small angle neutron scattering (SANS) to probe into the compositional and size evolution of the primary carbides and nanoparticles (defined by a size threshold of 100 nm) in an additively manufactured high-alloy steel before and after heat treatment. The primary carbides show a marginal change in their total volume fraction and size after the austenitising and tempering. However, primary carbides at the prior-austenite boundaries provide a pinning effect to restrict the grain growth during the austenitising. The grain size increases from 1.7 µm in the as-manufactured to 2.8 µm in the as-tempered condition, resulting in limited strength loss (estimated as 33-126 MPa by using the Hall-Petch relationship). Atom probe tomography (APT) which examines a sample volume of 6.6×105 nm3 but with atomic spatial resolution reveals the presence of many V and Cr-enriched nanoparticles with sizes of 1 to 10 nm in the steel matrix after the tempering. The complementary SANS which examines a significantly larger sample volume of 0.9 mm3 but without the spatial resolution provides the nanoparticle size information, revealing the radius change from 7.6 to 1.0 nm and volume fraction from 1.6% to 2.3%, in the as-manufactured and as-tempered conditions, respectively. The tempering induced nanoparticles can contribute to the strength enhancement of 691 MPa based on the Orowan bypass mechanism. It is thus believed that the limited prior-austenite grain growth, coupled with the nanoparticles, is the root cause for the unprecedented material strength.