Conventionally case hardening of steels is achieved by carburization at elevated temperatures to improve hardness and wear resistance using reactive atmospheres [1]. Alternatively, functionally graded metals can be fabricated by Laser Metal Deposition (LMD) by in-process powder blending [2,3]. Following this approach, investigations were carried out on additively manufactured carbon graded case hardening steel. Two powder feeders were loaded with pure 20MnCr5 and a blend of 20MnCr5 and FeC4.3 to achieve a nominal carbon content of 0.9 wt.‑%, enabling a layer-wise adjustment of the carbon concentration. Cubic samples of 10 × 10 × 5 mm were fabricated with different carbon content layer structures by LMD. To simulate subsequent conventional tempering, an additional laser was employed for in-situ localized reheating. Selected samples were exposed to different laser powers and durations on top surface immediately after deposition. Hardness measurements, light microscopy and WDX mapping were performed to characterize carbon profiles and their influence on microstructure and material properties.

The overall microstructure was much finer than after conventional case hardening due to higher cooling rates during LMD of 103 ‑ 104 K/s [4]. Martensitic microstructure was observed at carbon-rich areas, although a reduction of carbon of approx. 0.3 – 0.4 wt.‑% was detected compared to powder blend composition. The loss is reflecting partial carbon evaporation during the laser process [5]. Hardness profiles similar to conventional case hardening can be achieved. Tempering effects of the thermally metastable martensite were identified by microstructural investigations as root cause for the hardness decrease. Regions of pure 20MnCr5 remained unaffected. The findings highlight that LMD is capable to produce case hardened steel components with competitive graded properties.

References
[1] E. Wołowiec-Korecka Archives of Materials Science and Engineering, 2023, 120.2, 70-85.
[2] X. Chen; J. Chen; D. Zhan; S. Chen; J. Liang; M. Wang Materials Science & Engineering A, 2022, 856, 143931-143949.
[3] M. Yu; A. Feng; L. Yang; M. Erick Thomas Corrosion Science, 2021, 193, 109876-109883.
[4] W. Hofmeister; M. Griffith; M. Ensz; J. Smugeresky Journal of the Minerals Metals & Materials Society, 2001, 9, 30-34.
[5] N. Maharjan; W. Zhou; Y. Zhou; N. Wu Applied Physics A, 2018, 124, 682-690.