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Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion
Thesis (MEng)--Stellenbosch University, 2025.
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Stellenbosch : Stellenbosch University
2026
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| _version_ | 1867613843914489856 |
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| access_status_str | Open Access |
| author | Sepako, Motheo |
| author2 | Sacks, Natasha |
| author_browse | Sacks, Natasha Sepako, Motheo |
| author_facet | Sacks, Natasha Sepako, Motheo |
| author_sort | Sepako, Motheo |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (MEng)--Stellenbosch University, 2025. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/134817 |
| institution | Stellenbosch University (South Africa) |
| last_indexed | 2026-06-10T12:42:35.472Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2026 |
| publishDateRange | 2026 |
| publishDateSort | 2026 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/134817 Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion Sepako, Motheo Sacks, Natasha Mkhaliphi, Thuli Blaine, Deborah Stellenbosch University. Faculty of Engineering. Dept. of Industrial Engineering. Stainless steel -- Heat treatment Powder metallurgy Metals -- Mechanical properties Thesis (MEng)--Stellenbosch University, 2025. Sepako, M. 2025. Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion. Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/41839a37-b2aa-407f-ac7f-e966a7e1751c ENGLISH ABSTRACT: The study aimed to determine suitable processing parameters for producing fully dense 420 stainless steel (420SS) through Laser Powder Bed Fusion (L-PBF). The results showed the as-built density between 94.8 to 99.9 % was achieved with laser powers between 90 and 200 W and related volumetric energy densities (VED) ranging from 63.0 to 158 J/mm³. The L-PBF processing parameters identified for high-density, high-hardness parts were VED values from 84.6 to 158 J/mm³, which combined laser power settings between 145 and 200 W, and scanning speeds between 510 and 700 mm/s and hatch distance between 0.06 mm and 0.12 mm. These processing parameters resulted in high density between 98.9 to 99.9 % and hardness values between 523 to 608 HV. The chosen processing parameters were at VED of 136.6 J/mm³ combining laser power of 200 W, scanning speed at 680 mm/s and hatch distance of 0.07 mm. The as-built 420SS produced with the chosen parameters recorded a yield strength of 464 MPa, ultimate tensile strength of 1503 MPa, and elongation of 21 %. Fracture analysis of the as-built material showed a mixed mode of ductile and brittle fracture. The study extended to heat treatment; two schedules were used. The first schedule, +T-400 (tempering at 400 °C), significantly improved the microstructure, hardness, and mechanical properties of the L-PBF produced 420SS. It softened the martensitic structure, relieved internal stresses, and resulted in a more homogeneous microstructure with refined martensitic laths. The heat treatment reduced hardness anisotropy, with hardness values becoming more consistent across the build orientations. Post-tempering, tensile properties showed notable improvements, including increased yield strength to 587 MPa, tensile strength to 1535 MPa, and elongation to 25 %, with a predominantly ductile fracture surface observed. The second heat treatment schedule, +QT-560 °C, involved austenitising at 1000 °C in argon, gas quenching, and tempering at 560 °C. The formation of chromium-rich carbides during tempering contributed to improved hardness. The +QT-560 treatment resulted in significant increases in yield strength to 667 MPa compared to the as-built and +T-400 conditions; however, there was a noticeable decrease in the ultimate tensile strength to 890 MPa and elongation was reduced to 7.9 %. Fracture analysis revealed a mixed fracture mode, with both ductile and brittle characteristics, indicating a trade-off between high strength and reduced ductility. AFRIKAANSE OPSOMMING: Hierdie studie het beoog om geskikte verwerkingsparameters vir die produksie van ten volle digte 420 vlekvrye staal (420VS) deur laser- poeierbedsmelting (L-PBF) te bepaal. Die resultate het aangetoon dat soos-geboude digtheid van tussen 94.8 % en 99.9 % bereik is met lasersterkte van tussen 90 en 200 W en verwante volumetriese energiedigthede (VED) wat wissel van 63.0 tot 158 J/mm³. Die L-PBF-verwerkingsparameters wat vir hoëdigtheid-, hoëhardheidsonderdele geïdentifiseer is, was VED-waardes van 84.6 tot 158 J/mm³, wat laserkragstellings van tussen 145 en 200 W, en skandeerspoed tussen 510 en 700 mm/s en luikafstand tussen 0.06 mm en 0.12 mm gekombineer het. Hierdie verwerkingsparameters het hoë digtheid tussen 98.9 % en 99.9 % en hardheidswaardes van tussen 523 en 608 HV opgelewer. Die gekose verwerkingsparameters was teen VED van 136.6 J/mm3, wat laserkrag van 200 W, skandeerspoed van 680 mm/s en luikafstand van 0.07 mm gekombineer het. Die soos-geboude 420VS wat met die gekose parameters gelewer is, het ʼn swigsterkte van 464 MPa, breektreksterkte van 1 503 MPa, en rekking van 21 % aangeteken. Breukontleding van die soos-geboude materiaal het ʼn gemengde modus van rekbare en bros breuke getoon. Die studie het ook hittebehandeling ingesluit en twee skedules is gebruik. Die eerste skedule, +T 400 (temper teen 400 °C), het die mikrostruktuur, hardheid, en meganiese eienskappe van die L-PBF-geproduseerde 420VS beduidend verbeter. Dit het die martensitiese struktuur versag, interne spanning verlig en ʼn meer homogene mikrostruktuur met geaffineerde martensitiese latte opgelewer. Die hittebehandeling het hardheidsanisotropie verminder, met hardheidswaardes wat meer konsekwent oor die bou-oriëntasies heen geraak het. Daar was ʼn beduidende verbetering in trekeienskappe ná tempering, met inbegrip van verhoogde swigsterkte tot op 587 MPa, breektreksterkte tot op 1 535 MPa, en rekking tot op 25 %, met ʼn oorheersend rekbare breukoppervlakke wat waargeneem is. Die tweede hittebehandelingskedule, +QT-560, het austenitisering teen 1 000 °C in argon, gasafkoeling, en temper teen 560 °C behels. Die vorming van chroomryk karbiede tydens temper het tot verbeterde hardheid bygedra. Die +QT-560-behandeling het ʼn beduidende verhoging in swigsterkte tot op 667 MPa vergeleke met die soos-gebou en die +T-400-toestande opgelewer; daar was egter ʼn merkbare verlaging in die breektreksterkte tot op 890 MPa en rekking is na 7.9 % verminder. Breukontleding het ʼn gemengde breukmodus met rekbare en bros eienskappe getoon, wat dui op ʼn kompromis tussen hoë sterkte en verminderde rekbaarheid. Masters 2026-01-09T08:06:16Z 2026-01-09T08:06:16Z 2025-12 Thesis https://scholar.sun.ac.za/handle/10019.1/134817 Stellenbosch University xiii, 113 pages : illustrations application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Stainless steel -- Heat treatment Powder metallurgy Metals -- Mechanical properties Sepako, Motheo Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion |
| title | Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion |
| title_full | Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion |
| title_fullStr | Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion |
| title_full_unstemmed | Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion |
| title_short | Influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion |
| title_sort | influence of heat treatment on the microstructure and tensile properties of 420 stainless steel febricated by laser powder bed fusion |
| topic | Stainless steel -- Heat treatment Powder metallurgy Metals -- Mechanical properties |
| url | https://scholar.sun.ac.za/handle/10019.1/134817 |
| work_keys_str_mv | AT sepakomotheo influenceofheattreatmentonthemicrostructureandtensilepropertiesof420stainlesssteelfebricatedbylaserpowderbedfusion |