Creep behavior of 316 L stainless steel manufactured by laser powder bed fusion

Abstract

Additive manufacturing as a new processing technique can produce unique microstructure that is difficult to achieve using conventional techniques. In this study, we have investigated the creep behavior of 316 L stainless steel produced by a laser powder bed fusion process at temperatures of 550, 600 and 650 °C and stresses between 175 and 300 MPa. We found that additively-manufactured 316 L stainless steel had a higher stress dependence of the minimum creep rate than conventionally-made Type 316 SS, which could be attributed to the dislocation cell structure resulting from the printing process. The dislocation cell structure was unstable under creep, evolving into a uniform dislocation structure under the test conditions. While internal porosity in AM 316 L SS may serve as nucleation sites of creep voids and may be responsible for a relatively lower creep life, additively-manufactured 316 SS did not show inferior creep ductility when compared with conventionally-made 316 SS. The creep life of AM 316 L SS could be improved by stabilizing dislocation cell structure and/or reducing internal porosity through an optimized additive manufacturing process.

Introduction

Type 316 austenitic stainless steel has been widely used in various types of nuclear reactors, including current nuclear power plants and next-generation advanced reactor concepts, e.g. sodium-cooled fast reactors, molten-salt reactors, and gas-cooled high temperature reactors. Additive manufacturing as a disruptive manufacturing technology has opened up unprecedented opportunities for designing components with controlled microstructure, enabling smart designs of reactor components with complex geometries, design freedom and possibly enhanced properties [1]. Previous studies have shown that additively-manufactured 316 stainless steel (AM 316 SS) has significantly improved low-temperature tensile strength and good ductility over conventionally-made wrought 316 SS [2], [3], [4]. The high-temperature mechanical performance of AM 316 SS is yet to be evaluated and fully understood. A comprehensive database of elevated temperature mechanical properties is needed for its structural applications in the Transformational Challenge Reactor (TCR), a gas-cooled microreactor being developed to demonstrate revolutionary technologies including additive manufacturing [5,6].

Laser powder bed fusion (LPBF) is an additive manufacturing technology whereby laser beams are used to melt and consolidate metal powder lay-by-layer to form parts. It can fabricate complex components with high accuracy that is difficult to achieve by conventional manufacturing techniques, e.g. casting and forging. This new manufacturing technology has seen broad applications in a wide range of industries [7]. Today, austenitic stainless steels are routinely processed by LPBF [8]. The microstructure of LPBF-produced 316 SS is, however, much more complex than that of conventionally-made 316 SS. Various types of defects can be produced during the printing process, such as pores, cracks, etc. [8]. The high-energy input, rapid melting and solidification process result in a highly non-equilibrium microstructure. As has been observed previously, the microstructure of AM 316 L SS has a hierarchical microstructure with length scales spanning several orders of magnitude [3]. A melt pool contains columnar grains within which subgranular cellular structure prevails [9], [10], [11], [12]. Solidification cells have diameters of 1 µm or less and have similar crystallographic orientation. Cell walls contain high density dislocations and are segregated with chromium and molybdenum. The cell size is dependent on the thermal gradient and the solidification rate [3,9,12]. Rapid solidification has long been recognized as a useful means of producing dislocation substructures. Formation of dendrites and solute segregation resulting from rapid solidification promote the formation of dislocations [13]. Dislocations form cell structure under thermal stress during rapid solidification and cooling by a knitting process involving stress-induced climb [14,15].

The creep behavior of a material is generally determined by (1) material's intrinsic properties e.g. melting temperature, elastic modulus, diffusivity, (2) microstructural features (e.g. grains, subgrains, dislocations, second-phase precipitates), and (3) operating conditions (temperature, stress, environment). Dislocation substructures (cells, subgrains) play an important role in the creep resistance of a material. It should be noted that “dislocation cells” and “subgrains” have been traditionally used to describe the morphologies of dislocation substructures. Dislocation cells often refer to the dislocation substructure consisting of broad, diffused boundaries containing dislocation tangles, while subgrains refer to substructure with narrow and well defined sub-boundaries having a larger misorientation than cell walls. Sometimes these two terms are used interchangeably. In conventionally-made 316 SS, dislocation substructures are widely observed in creep tests at temperatures > 700 °C [16], [17], [18], [19], [20]. The influence of subgrain size on the creep resistance was also demonstrated by Sherby and Burke [21] who showed that the creep rate of aluminum with a fine subgrain size has higher creep resistance than a material containing coarse subgrains. In AM 316 SS, dislocation cell structure exists before creep tests, and is, in fact, a structural variable for creep. Little information is available in the literature regarding the creep resistance of AM 316 SS and the roles of the unique microstructural features, e.g. dislocation cells, porosity, solute segregation in the creep response of AM 316 SS.

The objective of this study is to evaluate the creep behavior of an LPBF-produced 316 L SS (referred to as “AM 316 L SS” throughout the paper) and to understand the effect of unique microstructural features of AM 316 L SS on its creep resistance. This understanding is vital to the optimization of process parameters and post-built treatments to produce high-quality parts with optimized microstructure and superior properties.