Standard martensitic stainless steels include types 403, 410, 414, 416, 416(Se), 420, 431, 440A, 440B, and 440C. The corrosion resistance of these steels is derived from their chromium content, which ranges from 11.5% to 18%. Steels with higher chromium content generally require a correspondingly higher carbon content to ensure the formation of martensite during heat treatment. The three aforementioned 440-series stainless steels are rarely considered for applications requiring welding, and filler metals with compositions matching the 440-series are not readily available.
Modified versions of standard martensitic steels incorporate alloying elements such as nickel, molybdenum, and vanadium. These additions primarily serve to raise the limited allowable service temperature of the standard steels to levels exceeding 1100 K. When these elements are added, the carbon content also increases; as the carbon content rises, the problem of cracking within the hardened heat-affected zone (HAZ) adjacent to the weld becomes more severe.
Martensitic stainless steels can be welded in the annealed, hardened, or hardened-and-tempered states. Regardless of the steel's initial condition, welding invariably results in the formation of a hardened martensitic zone adjacent to the weld bead. The hardness of this heat-affected zone is primarily determined by the carbon content of the base metal. As hardness increases, toughness decreases, rendering this region more susceptible to cracking. Preheating and controlling the interpass temperature are the most effective methods for preventing such cracking; furthermore, to achieve optimal mechanical properties, post-weld heat treatment is required. Martensitic stainless steels constitute a class of stainless steels whose properties can be tailored through heat treatment (specifically, quenching and tempering); colloquially, they are referred to as "hardenable" stainless steels.
This characteristic dictates that such steels must satisfy two fundamental conditions: First, their equilibrium phase diagrams must exhibit an austenitic phase region. This ensures that-after prolonged heating within the temperature range of this region to dissolve carbides into the steel matrix-subsequent quenching will result in the formation of martensite; in other words, the chemical composition must be controlled to fall within the γ (austenite) or γ+α (austenite-plus-ferrite) phase fields. Second, to enable the formation of a passive film that provides resistance to corrosion and oxidation, the chromium content must be maintained at or above 10.5%. Based on differences in alloying elements, these materials can be classified into martensitic chromium stainless steels and martensitic chromium-nickel stainless steels.
The primary alloying elements in martensitic chromium stainless steels are iron, chromium, and carbon. In the iron-rich region of the Fe-Cr binary phase diagram-specifically when the chromium content exceeds 13%-the gamma (γ) phase does not exist. Consequently, such alloys are single-phase ferritic alloys and cannot form martensite under any heat treatment regime. To overcome this, austenite-forming elements must be introduced into the Fe-Cr binary alloy system to expand the gamma-phase field. In the context of martensitic chromium stainless steels, carbon (C) and nitrogen (N) serve as effective elements; the addition of C and N allows the alloy to accommodate higher chromium contents.
Within the family of martensitic chromium stainless steels, carbon (C) stands-alongside chromium-as another indispensable and critical element. Indeed, martensitic chromium stainless heat-resistant steels are fundamentally a class of ternary Fe-Cr-C alloys. Naturally, other elements are also present; by taking these elements into account, the approximate microstructure of the alloy can be determined using the Schaeffler diagram. Martensitic stainless steels primarily consist of low-carbon or high-carbon steels with a chromium content ranging from 12% to 18%. The martensitic stainless steel grades widely utilized across various nations generally fall into the following three categories:
1. Low-carbon and medium-carbon 13% Cr steels
2. High-carbon 18% Cr steels
3. Low-carbon 17% Cr steels containing nickel (approximately 2%)
