Mechanical Properties of Materials
Mechanical properties of materials refer to the mechanical characteristics exhibited by materials under various environmental conditions (temperature, medium, humidity) when subjected to external loads (tension, compression, bending, torsion, impact, alternating stress, etc.).
The strength of stainless steel is determined by various factors, but **the most critical and fundamental factor lies in the different chemical elements added**, primarily metallic elements. Different types of stainless steel exhibit distinct strength characteristics due to differences in their chemical compositions.
01 Strength (Tensile Strength, Yield Strength)
Martensitic Stainless Steel
Martensitic stainless steel shares the hardening-through-quenching property of ordinary alloy steels, allowing a wide range of mechanical properties to be achieved by selecting grades and heat treatment conditions.
Martensitic stainless steel broadly belongs to the Fe-Cr-C system. It can be further classified into martensitic chromium stainless steel and martensitic chromium-nickel stainless steel. The trends in strength changes due to additions of chromium, carbon, and molybdenum in martensitic chromium stainless steel, as well as the strength characteristics of nickel additions in martensitic chromium-nickel stainless steel, are described below.
In martensitic chromium stainless steel under quenched and tempered conditions, increasing chromium content raises ferrite content, thereby reducing hardness and tensile strength. For low-carbon martensitic chromium stainless steel in annealed conditions, increasing chromium content slightly improves hardness while slightly decreasing elongation. At fixed chromium content, higher carbon content increases post-quench hardness but reduces plasticity. Molybdenum additions primarily enhance strength, hardness, and secondary hardening effects. After low-temperature quenching, molybdenum’s effect becomes particularly pronounced, with typical content below 1%.
In martensitic chromium-nickel stainless steel, a certain nickel content reduces δ-ferrite content, maximizing hardness.
The chemical composition of martensitic stainless steel typically ranges from 0.1%–1.0% carbon and 12%–27% chromium, with additions of molybdenum, tungsten, vanadium, and niobium. Its body-centered cubic structure causes rapid strength decline at high temperatures. However, below 600°C, it exhibits the highest high-temperature strength and creep resistance among stainless steels.
Ferritic Stainless Steel
Research shows that when chromium content is below 25%, ferritic structure suppresses martensite formation, leading to reduced strength with increasing chromium content. Above 25%, solid solution strengthening slightly improves strength. Higher molybdenum content promotes ferrite formation and the precipitation of α’, σ, and χ phases, enhancing strength via solid solution. However, this also increases notch sensitivity and reduces toughness. Molybdenum contributes more significantly to strength enhancement in ferritic stainless steel than chromium.
Ferritic stainless steel typically contains 11%–30% chromium with additions of niobium and titanium. It has the lowest high-temperature strength among stainless steels but offers the highest resistance to thermal fatigue.
Austenitic Stainless Steel
In austenitic stainless steel, increased carbon content enhances strength through solid solution strengthening.
Austenitic stainless steel is based on chromium and nickel, with additions of molybdenum, tungsten, niobium, and titanium. Its face-centered cubic structure provides high strength and creep resistance at elevated temperatures. However, its larger thermal expansion coefficient results in poorer thermal fatigue resistance compared to ferritic stainless steel.
Duplex Stainless Steel
Studies on the mechanical properties of duplex stainless steel with ~25% chromium show that increasing nickel content within the α+γ dual-phase region increases γ-phase content. When chromium content is 5%, yield strength peaks, and maximum strength is achieved at 10% nickel.
02 Creep Strength
Creep refers to the time-dependent deformation under sustained stress. At elevated temperatures, higher loads accelerate creep rates. Conversely, lower temperatures slow creep, with a threshold temperature below which creep becomes negligible. For pure iron, this threshold is ~330°C, while for stainless steel (due to strengthening measures), it exceeds 550°C.
Factors such as smelting, deoxidation, solidification, heat treatment, and processing significantly affect creep properties. For example, tests on 18-8 stainless steel in the U.S. revealed a standard deviation of ~11% in creep rupture time for samples from the same ingot, while samples from different ingots showed deviations twice as large. German tests on 0Cr18Ni11Nb steel demonstrated creep strengths ranging from <49 MPa to 118 MPa over 10⁵ hours, indicating substantial variability.
03 Fatigue Strength
High-temperature fatigue involves material failure under cyclic stress at elevated temperatures. Studies show that the 10⁸-cycle high-temperature fatigue strength is approximately half the high-temperature tensile strength at the same temperature.
Thermal fatigue occurs during heating (expansion) and cooling (contraction), where internal stresses develop due to constrained thermal deformation. Rapid thermal cycling generates shock-like stresses, potentially causing brittle failure (thermal shock). While thermal fatigue involves significant plastic strain, thermal shock primarily causes brittle fracture.
Composition and heat treatment affect high-temperature fatigue strength. Increased carbon content notably improves fatigue strength, as does solution heat treatment temperature. Ferritic stainless steels generally exhibit superior thermal fatigue resistance. Among austenitic grades, high-silicon steels with good high-temperature ductility perform best.
Materials with lower thermal expansion coefficients, smaller strain per thermal cycle, lower deformation resistance, and higher fracture strength exhibit longer fatigue life. For example, martensitic 1Cr17 stainless steel has the longest fatigue life, while austenitic grades like 0Cr19Ni9 and 2Cr25Ni20 have the shortest. Castings are more prone to thermal fatigue failure than forgings.
At room temperature, the 10⁷-cycle fatigue strength is roughly half the tensile strength. Fatigue strength shows minimal variation between room and elevated temperatures.
04 Impact Toughness
Impact toughness refers to the energy absorbed during impact loading. For cast maraging stainless steel, impact toughness is low at 5% nickel content. Increasing nickel improves strength and toughness, but values decline again above 8% nickel. Adding molybdenum to martensitic chromium-nickel stainless steel enhances strength without compromising toughness.
In ferritic stainless steel, higher molybdenum content increases strength but raises notch sensitivity, reducing toughness.
Austenitic chromium-nickel stainless steels with stable austenitic structures exhibit excellent toughness (both at room and cryogenic temperatures), making them suitable for diverse environments. Adding nickel to stable austenitic chromium-manganese steels further improves toughness.
In duplex stainless steel, impact toughness increases with nickel content, typically stabilizing between 160–200 J in the α+γ dual-phase region.