Impact of the Allowed Compositional Range of Additively Manufactured 316L Stainless Steel on Processability and Material Properties

This work aims to show the impact of the allowed chemical composition range of AISI 316L stainless steel on its processability in additive manufacturing and on the resulting part properties. ASTM A276 allows the chromium and nickel contents in 316L stainless steel to be set between 16 and 18 mass%, respectively, 10 and 14 mass%. Nevertheless, the allowed compositional range impacts the microstructure formation in additive manufacturing and thus the properties of the manufactured components. Therefore, this influence is analyzed using three different starting powders. Two starting powders are laboratory alloys, one containing the maximum allowed chromium content and the other one containing the maximum nickel content. The third material is a commercial powder with the chemical composition set in the middle ground of the allowed compositional range. The materials were processed by laser-based powder bed fusion (PBF-LB/M). The powder characteristics, the microstructure and defect formation, the corrosion resistance, and the mechanical properties were investigated as a function of the chemical composition of the powders used. As a main result, solid-state cracking could be observed in samples additively manufactured from the starting powder containing the maximum nickel content. This is related to a fully austenitic solidification, which occurs because of the low chromium to nickel equivalent ratio. These cracks reduce the corrosion resistance as well as the elongation at fracture of the additively manufactured material that possesses a low chromium to nickel equivalent ratio of 1.0. A limitation of the nickel equivalent of the 316L type steel is suggested for PBF-LB/M production. Based on the knowledge obtained, a more detailed specification of the chemical composition of the type 316L stainless steel is recommended so that this steel can be PBF-LB/M processed to defect-free components with the desired mechanical and chemical properties.

Effect of Chemical Composition on Structure and Corrosion Resistance of Ni-Mn-Cu Cast Iron

In the paper, a relationship between chemical composition of Ni-Mn-Cu cast iron and its structure, hardness and corrosion resistance is determined. The examinations showed a decrease of thermodynamic stability of austenite together with decreasing nickel equivalent value, in cast iron solidifying according to both the stable and the metastable systems. As a result of increasing degree of austenite transformation, the created martensite caused a significant hardness increase, accompanied by small decline of corrosion resistance. It was found at the same time that solidification way of the alloy and its matrix structure affect corrosion resistance to a much smaller extent than the nickel equivalent value, in particular concentration of elements with high electrochemical potential.

Effects of External and Internal Hydrogen on Tensile Properties of Austenitic Stainless Steels Containing Additive Elements

Effect of hydrogen on the slow strain rate tensile (SSRT) properties of five types of austenitic stainless steels, which contain small amounts of additive elements (e.g., nitrogen, niobium, vanadium and titanium), was studied. Some specimens were charged by exposing them to 100 MPa hydrogen gas at 543 K for 200 hours. The SSRT tests were carried out under various combinations of specimens and test atmospheres as follows: (i) non-charged specimens tested in air at room temperature (RT), (ii) non-charged specimens tested in 0.1 MPa nitrogen gas at 193 K, (iii) hydrogen-charged specimens tested in air at RT, (iv) hydrogen-charged specimens tested in 0.1 MPa nitrogen gas at 193 K, and (v) non-charged specimens tested in 115 MPa hydrogen gas at RT. In the tests without hydrogen (i.e., cases (i) and (ii)), the reduction of area (RA) was nearly constant in all the materials, regardless of test temperature. In contrast, in the tests of internal hydrogen (cases (iii) and (iv)), RA was much smaller at 193 K than at RT in all the materials. It was revealed that the susceptibility of the materials to hydrogen embrittlement (HE) can successfully be estimated in terms of the nickel equivalent, which represents the stability of austenite phase. The result suggested that the nickel equivalent can be used for evaluating the material compatibility of austenitic stainless steels for hydrogen service.

Effect of Nickel Equivalent on Austenite Transition Ratio in Ni-Mn-Cu Cast Iron

Determined was quantitative effect of nickel equivalent value on austenite decomposition degree during cooling-down castings of Ni-Mn- Cu cast iron. Chemical composition of the alloy was 1.8 to 5.0 % C, 1.3 to 3.0 % Si, 3.1 to 7.7 % Ni, 0.4 to 6.3 % Mn, 0.1 to 4.9 % Cu, 0.14 to 0.16 % P and 0.03 to 0.04 % S. Analysed were castings with representative wall thickness 10, 15 and 20 mm. Scope of the examination comprised chemical analysis (including WDS), microscopic observations (optical and scanning microscopy, image analyser), as well as Brinell hardness and HV microhardness measurements of structural components.

Investigation on Influence Factors of Mechanical Properties of Austenitic Stainless Steels for Cold Stretched Pressure Vessels

Cold stretched pressure vessels from austenitic stainless steels (ASS) have been widely used all over the world for storage and transportation of cryogenic liquefied gases. Cold stretching (CS) is performed by pressurizing the finished vessels to a specific pressure to produce the required stress which in turn gives an amount of plastic deformation to withstand the pressure load. Nickel equivalent (Nieq) and preloading, which is introduced in welding procedure qualification for cold stretched pressure vessels, are considered to be important factors to mechanical behavior of ASS. During the qualification, welded joint will be preloaded considering the effect of CS on pressure vessels. After unloading, the preloaded welded joint will go through tensile test according to standard requirements. There are two kinds of preloading method. One is to apply required tensile stress σk on specimen and maintain it for a long time (stress-controlled preloading). The other is to stretch specimen to a specific strain of 9% (strain-controlled preloading). Different preloading and preloading rates may lead to differences in mechanical behavior of preloaded welded joint. In order to understand the effects of nickel equivalent, preloading and preloading rate on the mechanical behavior of ASS for cold stretched pressure vessels, a series of tests were conducted on base metal, welded joint, and preloaded welded joint of ASS EN1.4301 (equivalent to S30408 and AISI 304). As regards to the preloaded welded joint, the ultimate tensile strength (UTS) decreased as the nickel equivalent increased, while the elongation to fracture increased. It was more difficult to meet the available mechanical requirements with strain-controlled preloading case than with stress-controlled preloading case. Rates of preloading had some effect on the mechanical properties of welded joint but nearly no effect on the mechanical properties of preloaded welded joint. These results are helpful for choosing appropriate material and determining a proper preloading method for welding procedure qualification.