Understanding the properties of austenitic stainless steels through the Schaeffler phase diagram

Austenitic stainless steel is simple from a metallurgical perspective. They are either 100% austenite or austenite containing a small amount of ferrite (see Table 1). This is not ferrite in carbon steel, but a high-temperature form of δ-ferrite. Unlike carbon steel and low-alloy steel, austenitic stainless steels do not undergo phase transformation when cooled at high temperatures. Therefore, they cannot be quenched and hardened to form martensite, and their mechanical properties are largely unaffected by welding. Therefore, hydrogen-induced cracks are not a problem, and preheating is not required regardless of the component thickness. The alloying elements in austenitic stainless steel can be divided into two categories: those that promote the formation of austenite and those that favor the formation of ferrite. Nickel, carbon, manganese, and nitrogen are the main elements that form austenite ； Important ferrite-forming elements are chromium, silicon, molybdenum, and niobium. By changing the content of these elements, steel can be fully austenitized, or designed to contain a small amount of ferrite. In 1949, Anton Schaeffler published a composition phase diagram showing the effect of composition on the microstructure. In the diagram, Schaeffler assigned a factor to various elements; this factor reflects the intensity of their influence on the formation of ferrite or austenite ； These factors can be seen in the graph. These elements are then combined into two groups to obtain the “equivalents” of chromium and nickel. They constitute the x-axis and y-axis of the graph, and by knowing the composition of austenitic stainless steel, it is possible to determine the proportion of the phases.

The Schaeffler diagram represents a quantitative relationship chart showing the chemical composition (excluding nitrogen) and microstructure of stainless steel weld metal. It is a microstructure diagram drawn by Schaeffler based on empirical statistics of the weld microstructure from manual arc welding of stainless steel (1949). Using this diagram, the effective methods for deposition can be determined. In the microstructure diagram, the vertical axis is represented by Nieq (nickel equivalent), which is an indicator reflecting the degree of austenitization in the microstructure of stainless steel weld metal. Its value is determined based on the austenite-forming elements present in the weld metal structure (such as nickel, carbon, manganese, etc.), and is converted into an equivalent amount expressed as a sum of certain amounts of nickel, depending on the intensity of their austenitizing effect ； The abscissa is represented by creq (chromium equivalent), which is an indicator of the degree of ferritization in the microstructure of the weld metal. Its value is determined by converting the amount of ferritizing elements present in the weld structure – such as chromium, molybdenum, silicon, niobium, etc. – into an equivalent amount of chromium, based on the intensity of their ferritizing effect. The diagram indicates the areas corresponding to various microstructures such as A (austenite), F (ferrite), M (martensite), etc. Based on the chemical compositions of the base material to be welded and the welding filler material, the chemical composition of the weld metal is determined using the fusion welding dilution factor, which is then converted into nickel equivalent and chromium equivalent. This allows it to be determined from the microstructure diagram what phases are present in the weld metal as well as its ferrite content. Conversely, the corresponding nickel equivalent and chromium equivalent values can also be determined based on the phase composition requirements of the weld metal structure. Then, the chemical composition of the weld metal is adjusted based on this organization diagram. The Schaeffler microstructure diagram takes into account the effect of chemical composition on the microstructure, but does not consider the influence of actual crystallization conditions and the form in which alloying elements exist. In fact, alloying elements only affect the ratio of γ-austenite to δ-ferrite when in a solid solution state. When alloying elements precipitate in the form of compounds, they do not affect the ratio of γ to δ. Different welding methods, welding processes, and joint configurations all affect the dilution rate in fusion welding as well as the solidification conditions of the weld pool. The δ-ferrite content estimated using the Schaeffler diagram differs from the measured value (the estimation accuracy is ±4% on a volume basis). Nevertheless, using this microstructure diagram in engineering to estimate the phase composition of stainless steel weld metal is relatively simple, and it still holds certain practical value.

The typical locations of common alloys are shown in the figure above. Colorful areas are also overlaid on the diagram to identify some of the problems that austenitic stainless steels may encounter. Although all austenitic stainless steels are sensitive to hot cracking, the fully austenitic steels (such as type 310) within the vertical blue area in Figure 1 are particularly sensitive. The culprits are sulfur and phosphorus. To this end, these impurity elements have been gradually reduced; as a result, steels with a sulfur content of less than 0.010% and a phosphorus content of less than 0.020% are now available at any time. Ideally, the sulfur and phosphorus content in 310 or 317 alloys should be below 0.003%. Cleaning is also extremely important; thorough degreasing must be carried out immediately before welding. Stains such as 304, 316, and 347 are located within or near the small uncolored triangular area at the center of the diagram; they contain a small amount of δ-ferrite, which although does not prevent thermal cracking, enhances resistance to the formation of sulfur-containing liquid films. The reason is that ferrite can dissolve more sulfur and phosphorus than austenite; as a result, they remain in the solution rather than forming liquid films along the grain boundaries ； — The presence of a considerable amount of ferrite increases the grain boundary area; as a result, any liquid film must diffuse over a larger area and can no longer form a continuous film. Austenitic steel does not have a 100% advantage. The problem that occurs in low-sulfur steel is a phenomenon known as \"post-casting variation\" or \"variable penetration\". When welding low-sulfur steel (

The reliability of welded joints in oil and gas transmission pipelines is an issue that cannot be ignored in petroleum engineering safety. High-strength, large-diameter pipeline steel allows for higher transmission pressures, thereby improving transmission efficiency; it also results in thinner pipe walls, which in turn reduces the consumption of welding materials and lowers the costs of oil and gas transmission pipelines. As the strength grade of pipeline steel increases, the tendency to develop welding defects at the welded joints grows, as does the tendency for welding residual stresses and stress concentrations. This can lead to a reduction in fatigue strength, as well as an increased likelihood of failures such as welding hydrogen-induced cracks, brittle fracture, and stress corrosion cracking. Traditional methods for eliminating welding residual stresses are difficult to implement during on-site pipeline welding and inevitably increase costs. Reasonably controlling the distribution and magnitude of residual stress in pipeline steel weld joints is an important method to ensure the safety of welded structures. The special welding filler material method can utilize the volume expansion resulting from the phase transformations that occur during welding to compensate for the volume contraction caused by thermal shrinkage, thereby generating residual compressive stresses in the weld. This method does not require pre-welding or post-welding heat treatment; it enables the reduction or even elimination of residual stresses directly during the welding process. In addition to its low cost, it is particularly suitable for large and complex structures where post-welding heat treatment is difficult, as well as for welding operations in harsh conditions, offering broad application prospects. By combining theoretical and experimental methods, the role of alloying elements in steel and their influence on the martensite transformation temperature were analyzed. Three types of welding wires with different martensite transformation temperatures were developed based on the Schaeffer phase diagram; these are referred to as low-temperature transformation welding wires (LTT), and are named LTT1, LTT2, and LTT3 respectively. Using X65 pipeline steel as the base material, homogeneous welding wire and low-temperature phase transformation welding wire as filler materials, welding was carried out by manual tungsten inert gas arc welding (TIG). After welding, the microstructure of the welded joint was examined using an optical microscope (OM), a scanning electron microscope (SEM), and X-ray diffraction (XRD). The phase composition in the welded joint, as well as the diffusion of alloying elements and the morphology of the microstructure, were analyzed. The results showed that the microstructure of the welded joint resulting from low-temperature phase transformation consists mainly of martensite along with a small amount of residual austenite (10%~15%). Good fusion occurred between the base material and the weld metal; there was diffusion of alloying elements at the interface between them, with the levels of these elements showing a continuous variation in the fusion zone and the weld area. The low-temperature impact toughness, microhardness, and tensile strength of the welded joints were tested. The results showed that the microhardness and tensile strength of the weld metal in the low-temperature transformation welds were higher than those of the homogeneous filler wires. At room temperature, all three types of low-temperature transformation filler wires exhibited good impact toughness; however, as the temperature decreased, the impact toughness of the LTT1 and LTT3 welded joints declined significantly, whereas the impact toughness of the LTT2 welded joints remained relatively stable with changes in temperature. The residual stress state of welded joints was examined using the blind hole method. The results showed that in homogeneous welded joints, a relatively large amount of residual tensile stress exists in the weld area, with longitudinal peak values reaching up to 450 MPa and transverse peak values around 250 MPa. In welded joints subjected to low-temperature phase transformation, the stress state at the center of the weld is either residual compressive stress or a smaller amount of residual tensile stress, with longitudinal stresses ranging from -300 to 200 MPa and transverse stresses ranging from -200 to 200 MPa. Furthermore, the direction of deformation in the steel plates filled with homogeneous welding wire after welding is opposite to that in those filled with low-temperature phase-change materials. The degrees of deformation are as follows: the deformation of the welded joint after LTT1 welding is 0.056 rad, that of the welded joint after LTT2 welding is 0.083 rad, and that of the welded joint after LTT3 welding is 0.033 rad; whereas the deformation of the welded joint after using homogeneous welding wire is 0.14 rad. The main reason for this result is the change in the residual stress state. The corrosion resistance of welded joints filled with a low-temperature phase transition material was tested using electrochemical methods in two sets of 0.2 mol/L H2S saturated solutions with different NaCl concentrations and pH values. The results showed that as the NaCl concentration increased, the corrosion resistance of the welded joints decreased; whereas as the pH value of the solution increased, their corrosion resistance improved. When the solution shifted from acidic to slightly alkaline, the corrosion behavior of the welded joints in the corrosive medium was hardly affected by the pH value of the solution.