Corrosion Testing of a Heat Treated 316 L Functional Part Produced by Selective Laser Melting

Abstract

Selective Laser Melting (SLM) shows a big potential among metal additive manufacturing (AM) technologies. However, the large thermal gradients and the local melting and solidification processes of SLM result in the presence of a significant amount of residual stresses in the as built parts. These internal stresses will not only affect mechanical properties, but also increase the risk of Stress Corrosion Cracking (SCC). A twister used in an air extraction pump of a condenser to create a swirl in the water, was chosen as a candidate component to be produced by SLM in 316 L stainless steel. Since the main expected damage mechanism of this component in service is corrosion, corrosion tests were carried out on an as-built twister as well as on heat treated components. It was shown that a low temperature heat treatment at 450˚C had only a limited effect on the residual stress reduction and concomitant corrosion properties, while the internal stresses were significantly reduced when a high temperature heat treatment at 950˚C was applied. Furthermore, a specific stress corrosion sensitivity test proved to be a useful tool to evaluate the internal stress distribution in a specific component.

Corrosion of Additively Manufactured Stainless Steels—Process, Structure, Performance: A Review

“…Distribution of corrosion studies in terms of stainless steel alloy, process, and environment. [2][3][4][5][6][7]10,[18][19]27,29,37,[47][48][49][50][51][53][54]57,[68][69]76,[91][92][101][102][103][104][105]109,116,129,134,137,144,[146][147][148][149]153,178,[185][186][187][188][189][190][191][192][193][194]…”

“…DeBruycker reported severe cracking (multiple mm-sized cracks) of an asbuilt SLM 316L turbine part after 3 h in a boiling magnesium chloride solution, Figure 17. 131 Stress relieving the parts for 2 h at 450°C minimized crack frequency. Annealing at 2 h at 950°C, which provided the greatest stress relief, eliminated cracks even after 78 h immersion.…”

“…as-built SLM 316L part after 3 h immersion in boiling magnesium chloride solution. 131 The outer part surface is seen at the top of (a), including a partially fused powder particle. Reprinted from De Bruycker, et al 131…”

Corrosion of Additively Manufactured Alloys: A Review

“…The tabulation of such studies are presented in Table 4. 61,64,[73][74][75][76][77]80,[82][83]86,[95][96][97][98][99][100][101][102][103][104] Environments containing chloride ions are well known to be detrimental to the durability of stainless steels, with a number of studies tabulated above appropriately focusing on such electrolytes. 105 In a study by Ganesh, et al, 95 two batches of specimens (termed "A" and "B") were produced via DLD, by utilizing similar parameters to produce Type 316L specimens.…”

“…Heat treatments were performed in SLM 316L specimens to investigate SCC in boiling MgCl 2 (ASTM G36) in the work presented by Bruycker, et al 98 The increase of heat-treatment temperature from 450°C to 950°C increased the resistance to SCC of SLM 316L based on visual inspection of cracks formed on the analyzed specimen. The SLM 316L heat treated at 950°C did not present visual cracks after SCC testing.…”