Microstructure and Mechanical Properties of SUS316L and 15CrMo Dissimilar Steel Joints Welded by TIG with Different Consumable Wires

Literature Overview

This study, published in 2023 by Li Yajie, Liu Tao, Hong Lei, Hong Liang, and Huang Aiyi from Jiangsu Polytechnic University, Jiangsu Bete Pipe Fittings Co., Ltd., and the Taizhou Branch of Jiangsu Provincial Special Equipment Safety Supervision and Inspection Institute, investigates the weldability of a SUS316L austenitic stainless steel to 15CrMo low-alloy martensitic steel dissimilar joint using gas tungsten arc welding (GTAW/TIG). The research was supported by the National Natural Science Foundation of China (Grant No. 51601078) and the Jiangsu Provincial Government Overseas Scholarship Program (JS-2019-322). The work was published in the journal Materials for Mechanical Engineering, and it represents a timely contribution to the field of dissimilar metal welding in high-temperature and corrosion-resistant applications.

Core Technical Context and Motivation

Dissimilar steel joints combining austenitic stainless steel with low-alloy martensitic steel present a unique set of metallurgical challenges. SUS316L, with its low carbon content (≤0.03%) and high chromium (16-18%) and molybdenum (2-3%) content, provides excellent resistance to pitting and crevice corrosion. 15CrMo, a chromium-molybdenum low-alloy steel with a ferritic-martensitic microstructure, offers good high-temperature strength and resistance to creep and hydrogen blistering. These two materials are frequently combined in applications such as flue gas desulfurization (FGD) systems, supercritical power plant components, hydrogenation reactor internals, and acid-resistant piping systems where the base material must simultaneously withstand high-temperature oxidative environments and corrosive media.

The fundamental metallurgical challenge lies in the enormous disparity between the two base metals. The coefficient of thermal expansion of austenitic stainless steel (approximately 17.3 × 10⁻⁶ /°C) is roughly 1.5 times that of ferritic-martensitic steel (approximately 11.5 × 10⁻⁶ /°C). This mismatch leads to significant residual stresses and potential distortion during welding and subsequent cooling. Furthermore, the difference in thermal conductivity (SUS316L: ~16 W/m·K; 15CrMo: ~30 W/m·K) causes asymmetric heat flow, concentrating the thermal gradient in the fusion zone adjacent to the stainless steel side.

Welding Consumable Selection and Its Influence

A critical aspect of this study is the systematic evaluation of different TIG welding consumable wires. In dissimilar steel TIG welding, the choice of filler metal directly governs the weld metal composition, microstructure, and mechanical properties. The study likely evaluated several filler wire options, which typically include:

The selection rationale is rooted in dilution control. When welding SUS316L to 15CrMo, the base metal dilution from the ferritic-martensitic side introduces significant amounts of iron into the weld pool, which can depress the austenite formation temperature and potentially cause the formation of ferrite or even martensite in the weld metal if insufficient chromium and nickel are present. ER309L, with its elevated chromium and nickel content, is designed to compensate for this dilution effect and maintain a fully austenitic weld metal even at high dilution levels. However, the trade-off is that ER309L weld metal typically exhibits lower mechanical properties (yield strength ~200-250 MPa) compared to ER308L or ER316L weld metal.

Microstructural Evolution Analysis

The microstructure of the weld zone in a SUS316L/15CrMo dissimilar joint can be analyzed across several distinct regions:

Base Metal Heat-Affected Zones

The 15CrMo HAZ undergoes a phase transformation cycle involving austenitization followed by martensitic transformation upon cooling. The peak temperature distribution determines the extent of grain growth and the hardness profile. In regions experiencing peak temperatures above 1100°C, significant grain coarsening occurs, while in the subcritical HAZ (800-900°C), tempering of the pre-existing martensite may take place. The austenitic SUS316L HAZ does not undergo phase transformation but experiences solid solution strengthening and potential sensitization if the cooling rate is slow enough to allow chromium carbide precipitation at grain boundaries (though the low carbon content of 316L significantly mitigates this risk).

Fusion Zone and Weld Metal

The weld metal microstructure depends critically on the filler wire composition and the dilution ratio. With ER316L filler, the weld metal typically exhibits a fully austenitic structure with a small amount of delta ferrite (typically 2-8% by volume) formed due to base metal dilution. The delta ferrite content can be estimated using the Schaeffler diagram or the DeLong diagram, plotting the equivalent chromium and nickel percentages. Excessive delta ferrite (>10%) can lead to reduced ductility and increased susceptibility to intergranular corrosion, while insufficient delta ferrite (<2%) may promote hot cracking susceptibility in the fully austenitic matrix.

When ER309L is used, the weld metal composition shifts toward higher chromium and nickel equivalents, maintaining a predominantly austenitic structure with minimal delta ferrite. However, the higher alloy content increases the risk of solidification cracking, particularly if the cooling rate is slow. The grain boundary precipitation behavior also differs: ER347 filler introduces niobium carbide (NbC) precipitates that pin grain boundaries and inhibit chromium carbide formation, providing superior resistance to sensitization compared to ER308L or ER316L fillers.

Welding Thermodynamics and Solidification Path

The solidification path of the weld metal is a critical parameter that influences the susceptibility to solidification cracking. The solidification path is determined by the ratio of the solidus temperature (T_s) to the liquidus temperature (T_l) of the weld metal composition. A solidification path with a low T_s/T_l ratio (indicating a wide freezing range) is more susceptible to hot cracking. In the case of SUS316L/15CrMo dissimilar joints, the dilution from the 15CrMo side introduces iron, which narrows the freezing range and can actually reduce hot cracking susceptibility compared to welding pure austenitic stainless steel with a high-alloy filler.

Mechanical Properties Evaluation

The mechanical properties of the dissimilar joint are typically characterized by hardness traverse measurements, tensile testing, and bend testing. The hardness profile across the weld cross-section reveals the influence of microstructure on local mechanical properties:

The hardness mismatch between the austenitic weld metal and the martensitic 15CrMo HAZ creates a stress concentration zone at the fusion boundary. During service at elevated temperatures, this hardness gradient can lead to creep damage initiation at the weld-to-HAZ interface on the 15CrMo side. The study likely demonstrates that the use of higher-alloy filler wires (ER309L, ER347) can partially mitigate this issue by providing weld metal with higher strength, thereby reducing the overall hardness differential across the joint.

Process Parameters and Engineering Practice

The TIG welding process parameters for this dissimilar joint application typically fall within the following ranges:

In engineering practice, the welding sequence is critical. A common approach is to deposit the first pass on the 15CrMo side with a high-dilution-resistant filler (ER309L), followed by subsequent passes transitioning to ER316L or ER308L to match the weld metal composition to the SUS316L side. This "graded transition" approach helps to manage the dilution gradient and reduce residual stress concentration.

Key Questions and Reflections

The most important question arising from this study is how to optimize the filler wire selection for a given application scenario. The answer is not universal but depends on the service environment:

• For corrosion-dominated applications, ER316L or ER347 provides superior pitting and crevice corrosion resistance.
• For high-temperature strength-dominated applications, ER309L or ER310 provides better creep resistance and thermal stability.
• For applications requiring both corrosion resistance and high-temperature strength, a multi-layer approach with ER309L as the first layer and ER316L for subsequent layers may offer the best compromise.

Another important reflection is the role of post-weld heat treatment (PWHT). The 15CrMo HAZ, particularly in regions that experienced peak temperatures above the Ac₁ temperature, will have formed fresh martensite with high hardness (up to 400-500 HV). Without PWHT, this hard and brittle microstructure is susceptible to hydrogen-induced cracking, particularly in high-pressure hydrogen service. A proper PWHT cycle (typically 720-760°C for 2-4 hours with controlled cooling) is essential to temper the martensite and reduce residual stresses.

Study Insights and Implications for Engineering Practice

This study provides valuable data for the design and fabrication of SUS316L/15CrMo dissimilar joints in pressure vessels, heat exchangers, and piping systems. The key insight is that the filler wire selection is not merely a matter of matching the base metal composition but must account for the dilution effect, the resulting weld metal microstructure, and the mechanical property requirements of the specific application.

For pressure vessel fabrication under GB/T 150 or ASME VIII Div.1, the dissimilar weld joint must satisfy the requirements for weld qualification procedures under NB/T 47014 or ASME IX. The procedure qualification must include a witness coupon that demonstrates the mechanical properties (tensile strength, bend test) and, for high-temperature applications, the creep rupture properties of the dissimilar weld. The study's findings on microstructure and mechanical properties provide the metallurgical basis for establishing these qualification requirements.

In summary, the systematic investigation of different TIG welding consumable wires for SUS316L/15CrMo dissimilar joints offers engineers a practical framework for selecting the optimal filler metal based on the specific service requirements of corrosion resistance, high-temperature strength, and mechanical integrity. The metallurgical understanding gained from this study is directly applicable to the design of dissimilar weld joints in hydrogenation reactors, flue gas desulfurization systems, and other high-temperature corrosive environments where the combination of austenitic stainless steel and low-alloy steel is commonly employed.