Solidification behaviour of austenitic stainless steels during welding and directed energy deposition

Abstract

The effect of cooling rate on the solidification behaviour of austenitic stainless steels during high energy density welding and directed energy deposition (DED) has been reviewed. Precedent studies demonstrated the confinement of austenite–ferrite duplex region and the susceptibility of specific alloy compositions on the Schaeffler diagram to alteration of solidification mode at high cooling rates during the high energy density welding. Meanwhile, mitigated cooling conditions have dominated during the DED process. The instances of microstructural fluctuations owing to cooling rate variation have been compiled. The incorporation of DED steels into the implicated Schaeffler diagrams demonstrated reliable predictions at high cooling rates. The printability of austenitic stainless steels during the DED process has been discussed in terms of solidification cracking susceptibility.

KEYWORDS:

• Welding
• additive manufacturing
• directed energy deposition
• stainless steels
• solidification
• Schaeffler diagram
• microstructure

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1 All stainless steel compositions comply with the American Iron and Steel Institute (AISI) designation system throughout this paper.

2 Empirical relations such as d = 80(Ṫ)^−0.33 have been obtained in stainless steels gas tungsten arc (GTA) and tungsten inert gas (TIG) weld metals between measured dendritic arms spacing (d) and experimental cooling rates (Ṫ) from temperature measurement by thermocouple. Owing to difficulties with temperature measurement in tiny melt pools of high energy density welding and MAM, the empirical relation is often extrapolated at high cooling rates. Both primary dendritic arms spacing (PDAS) and secondary dendritic arms spacing (SDAS) have been applied for dendritic microstructures with different constants, and primary cellular arms spacing (PCAS) has been implemented for cellular microstructures.

3 These modes are applied in this paper, but the term primary austenite is applied to emphasize the solidification mode with austenite as the primarily solidifying phase (the leading one) irrespective of any secondarily solidifying ferrite whenever A and AF modes are not to be discerned essentially. Similarly, the term primary ferrite will be sued with no discernment between FA and F modes to emphasize primarily solidifying ferrite irrespective of any secondarily solidifying austenite. Noteworthy, those terms have occasionally been used for A and F modes in the literature. For a schematic corresponding to the present terminology see [51].

4 Schaeffler equivalencies: Cr_eq = Cr+Mo+1.5Si+0.5Nb and Ni_eq = Ni+0.5Mn+30C, in wt-%. Unless otherwise specified, the Schaeffler equivalencies apply hereafter.

5 Cr_eq = Cr+Mo+1.5Si+0.5Nb+2Ti and Ni_eq = Ni+0.5Mn+30C+30(N-0.06), in wt-%.

6 Cr_eq = Cr+1.37Mo+1.5Si+2Nb+3Ti and Ni_eq = Ni+0.31Mn+22C+14.2N+Cu, in wt-%.

7 Cr_eq = Cr+Mo+1.5Si+0.5Nb; Ni_eq = Ni+0.5Mn+30C+30N, in wt-% (DeLong equivalencies)