Microstructures in Austenitic Stainless Steels

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Introduction to Austenitic Stainless Steels

Stainless steels represent a specialized group of high-alloy steels based primarily on the Fe-Cr, Fe-Cr-Ni, and Fe-Cr-C systems. Among these, austenitic stainless steels constitute a particularly important category widely utilized across numerous industries and environments. The standard austenitic composition typically features approximately 18% chromium and 8% nickel.

While stainless steels are generally considered weldable materials, proper fabrication requires adherence to specific guidelines to ensure defect-free construction and expected service performance. Welding processes invariably cause significant microstructural changes in both the weld metal and heat-affected zone compared to the base metal. The susceptibility to weld solidification and liquation cracking depends primarily on the composition of both base and filler metals, with particular sensitivity to impurity elements such as sulfur and phosphorus.

High-Temperature Microstructural Transformations

Austenitic steels experience distinctive microstructural changes during exposure to elevated temperatures, with transformation patterns differing between short-term and long-term exposure. Research by Heino et al. demonstrated that during heat treatment between 600-900°C, short-duration precipitation from austenite (less than 60 seconds) typically results in the formation of M23C6 carbide. When aging periods extend longer, additional precipitates including intermetallic phases form, generally accompanied by the dissolution of existing carbides.

These intermetallic precipitations hold particular significance not only for their considerable influence on mechanical properties but also for their pronounced effect on corrosion resistance. Numerous studies have investigated the microstructural behavior and precipitation patterns of stainless steels during welding and exposure to high temperatures.

Microstructure Stability in Austenitic Stainless Steels

Microstructural stability represents one of the most critical requirements for establishing appropriate mechanical and corrosion properties in Austenitic Stainless Steels (ASSs). To achieve stable microstructures, these steels typically undergo solution heat treatment followed by annealing at temperatures between 700–1100 K. During the annealing process, secondary phases precipitate from the austenite (matrix phase with a face-centered cubic crystal lattice) and/or the δ-ferrite (high-temperature phase with a body-centered cubic crystal lattice).

Table 1. The secondary phases in austenitic stainless steels (Note: concentrations of non-metallic elements are not included in this table)

Phase/Crystal Lattice	Lattice Parameter (nm)	Fe (%)	Cr (%)	Ni (%)	Mo (%)	Si (%)	Steel
σ-phase, tetragonal	a = 0.883	49-52	33-34	4-7	1	8-11
							AISI 316
	a = 0.461	52	38	5	10	1	19Cr-12Ni-0.02C
	a = 0.883	52	38	4-7	10	1	AISI 316
		19	44-48	2-6	4-12	-	20Cr-25Ni-4Mo
Laves, hexagonal	a = 0.475	37	11	1	-	-	AISI 316
	a = 0.4773	35	11	1	-	-	AISI 316
	a = 0.473	38	4	3	43-47	-	316-LN-3
χ-phase, cubic	a = 0.890	51-53	23-24	3-4	18	1	AISI 316
	a = 0.890	51	24	4	18	1	316-LN-3
		51	23-24	4	18	1	UNS S31803
		51-53	24	4	18	1	AISI 316A30X
M23C6, cubic	a = 1.0905	14	17	1	4	-	19Cr-12Ni-25Si-0.02C
		16-23	18	1	4	-	20Cr-25Ni-4Mo
MX, cubic	a = 0.422	-	-	-	-	-	Ti = 99, AISI 321
Cr2N, hexagonal	a = 0.4776	100	-	-	-	-	316-LN-3
Z-phase, tetragonal	a = 1.00	-	23-24	3-4	7	1	20Cr-25Ni-4Mo
R-phase, cubic	a = 1.093
		55-58	6-7	-	20	1	UNS S31803
R-phase, hexagonal	a = 1.934	23-27	5	-	25	-	20Cr-25Ni-4Mo

In ASSs containing higher bulk carbon content (typically 0.3-0.6% by mass), the orthorhombic M₇C₃ carbide may also develop. Specialized ASSs with elevated silicon or phosphorus content may additionally form silicides (G-phase with a body-centered cubic crystal lattice) and phosphides (M₃P with a tetragonal crystal lattice). Some industrial applications utilize ASSs in their as-cast condition, such as centrifugally cast or sand-cast large tubes and elbows for pipeline systems. Given the highly heterogeneous microstructure of as-cast steels, understanding the proportion, chemical composition, and stability of phases (particularly δ-ferrite) becomes essential.

Characterization Techniques for Phase Identification

Information about phases present in ASSs can be obtained through both experimental and theoretical approaches. Modern phase identification employs precise experimental techniques including transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS), and electron diffraction. Additional techniques include Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM–EDX), Wavelength Dispersive X-ray diffraction (WDX), and Differential Thermal Analysis (DTA). These experimental methods are increasingly combined with thermodynamic predictions of phase equilibrium and modeling of phase evolution.

Figure 1: Main transformations that occur in austenitic stainless steels between room temperature and the liquid state.

Conclusion

The microstructural evolution of austenitic stainless steels during heat treatment and service conditions significantly affects their performance characteristics. Understanding these transformations, particularly the formation of secondary phases, provides critical insights for controlling material properties in various applications. The combination of advanced characterization techniques with theoretical models offers comprehensive tools for predicting and managing microstructural changes in these important engineering materials.