Martensitic Stainless Steels

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Composition and Basic Characteristics of Martensitic Stainless Steels

Martensitic stainless steels share structural similarities with low alloy and carbon steels, featuring a body-centered tetragonal (bct) crystal structure. Their distinctive properties emerge from a carefully balanced composition, with chromium (12-15%) serving as the primary alloying element, supplemented by molybdenum (0.2-1%), and carbon (0.1-1.2%). Unlike most stainless steel variants, these grades typically contain no nickel, with only two specialized grades being exceptions to this rule.

In their annealed state, these steels exhibit moderate yield strengths of approximately 275 MPa, making them suitable for machining, cold forming, or cold working operations. Their most notable characteristic is the ability to be significantly hardened through heat treatment, with the final strength directly correlating to the carbon content. While higher carbon concentrations enhance strength and hardness potential, they conversely reduce ductility and toughness. Premium grades can achieve impressive hardness ratings of up to 60 HRC through appropriate heat treatment protocols.

Heat Treatment and Transformation Characteristics

The optimization of martensitic stainless steels' properties relies heavily on precise heat treatment procedures, primarily involving hardening and tempering processes. The standard hardening treatment begins with austenitizing, where the steel is heated to temperatures between 925-1070°C. This critical step creates an austenitic structure with carbon in solid solution. The specific austenitizing temperature and duration significantly influence the final properties, particularly in relation to the steel's carbon content.

During the austenitizing process, the material's hardness typically increases with temperature until reaching a maximum threshold, after which it begins to decrease. Extended exposure to austenitizing temperatures generally results in a gradual reduction in hardness. Following austenitization, the material undergoes quenching - a process that can be performed in air, oil, or water, depending on the specific grade requirements.

The martensitic transformation occurs as the material cools below the MS-temperature (martensite start temperature), which ranges from 300-700°C. This transformation continues until approximately 150-200°C below the MS-temperature. It's noteworthy that most alloying elements, particularly carbon, lower the MS-temperature. In highly alloyed grades, this can result in retained austenite in the final microstructure, as the transformation completion temperature may fall below ambient conditions.

The martensitic transformation occurs as the material cools below the MS-temperature (martensite start temperature), which ranges from 300-700°C. This transformation continues until approximately 150-200°C below the MS-temperature. It's noteworthy that most alloying elements, particularly carbon, lower the MS-temperature. In highly alloyed grades, this can result in retained austenite in the final microstructure, as the transformation completion temperature may fall below ambient conditions.

Figure 1: The microstructure image of martensitic stainless steel

Tempering and Mechanical Properties

The as-hardened condition of martensitic stainless steels, while providing high strength and hardness, results in limited ductility and toughness. To achieve optimal engineering properties, tempering is essential. The tempering temperature selection critically influences the final mechanical characteristics of the steel, creating a precise balance between strength and ductility.

At tempering temperatures below 400°C, the changes in mechanical properties are relatively subtle. These include:

• Minor decreases in tensile strength
• Slight increases in reduction of area
• Minimal effects on hardness, elongation, and yield strength

A distinctive feature occurs around 450-500°C, where these steels exhibit a secondary hardening peak. During this temperature range:

• Both yield and tensile strength increase notably
• Hardness shows a marked improvement
• Impact toughness typically experiences a temporary decrease

Figure 2: Effect of tempering temperature on mechanical properties of AISI 431

Beyond 500°C, the mechanical properties undergo significant changes:

• Rapid reduction in strength and hardness
• Substantial increase in ductility and toughness
• When tempering exceeds 780°C (referring to the steel shown in Figure 2), partial austenitization may occur, potentially resulting in untempered martensite upon cooling

Table 1. The comprehensive composition table of AISI standard martensitic grades

AISI grade	C	Mn	Si	Cr	Ni	Mo	P	S	Comments/Applications
410	0.15	1	0.5	11.5-13.0	-	-	0.04	0.03	The basic composition. Used for cutlery, steam and gas turbine blades and buckets, bushings.
416	0.15	1.25	1	12.0-14.0	-	0.6	0.04	0.15	Addition of sulphur for machinability, used for screws, gears etc. 416 Se replaces suplhur by selenium.
420	0.15-0.40	1	1	12.0-14.0	-	-	0.04	0.03	Dental and surgical instruments, cutlery.
431	0.2	1	1	15.0-17.0	-	1.25-2.00	0.04	0.03	Enhanced corrosion resistance, high strength.
440A	0.60-0.75	1	1	16.0-18.0	-	0.75	0.04	0.03	Ball bearings and races, gage blocks, molds and dies, cutlery.
440B	0.75-0.95	1	1	16.0-18.0	-	0.75	0.04	0.03	As 440A, higher hardness.
440C	0.95-1.20	1	1	16.0-18.0	-	0.75	0.04	0.03	As 440B, higher hardness.

This careful balance of tempering parameters allows metallurgists to tailor the properties of martensitic stainless steels to specific application requirements, from high-strength components to moderately ductile parts.

Enhanced Compositions and Corrosion Resistance

Modern developments in martensitic stainless steels have focused on improving their performance through strategic alloying additions. Several advanced grades have been engineered specifically for moderately high-temperature applications, incorporating key elements such as:

• Molybdenum (Mo)
• Vanadium (V)
• Niobium (Nb)
• Limited quantities of nickel (up to 2 wt%) for enhanced toughness

These modifications have led to more complex precipitation sequences, particularly beneficial in power generation applications, where components like steam turbine blades operate at temperatures around 600°C.

Nitrogen addition has emerged as a significant advancement in improving localized corrosion resistance.

Research has shown that:

• Nitrogen additions up to 0.2 weight percent significantly improve resistance to intergranular corrosion, particularly in sulfuric acid environments
• High-temperature nitrided AISI 410S demonstrates superior corrosion resistance compared to conventional AISI 420, maintained across various tempering temperatures (200°C, 400°C, and 600°C)
• Corrosion-erosion resistance of high-nitrogen variants (AISI 410N, 410SN) surpasses traditional AISI 420 across temperatures ranging from 0°C to 70°C

Figure 3: The polarization curves comparing AISI 420 and high-temperature nitrided AISI 410S steels

This enhanced performance is attributed to nitrogen's beneficial effect when present in solid solution within the martensitic structure. While these steels still exhibit lower corrosion resistance compared to austenitic and ferritic grades, these improvements have expanded their application range significantly.

Industrial Applications and Practical Considerations

Martensitic stainless steels possess the highest strength but lowest corrosion resistance among the stainless steel families. Despite their lower corrosion resistance compared to austenitic and ferritic grades, these steels maintain good weldability. Their unique combination of high strength and moderate corrosion resistance makes them particularly suitable for applications requiring both corrosion and wear resistance. High-carbon variants are frequently employed as tool steels.

Typical applications include:

• Aerospace components
• Automotive parts
• Hydroelectric engines
• Cutlery
• Defense equipment
• Power hand tools
• Pump parts
• Valve seats
• Chisels
• Bushings
• Ball bearings
• Sporting equipment
• Surgical instruments

For optimal performance, these steels require careful consideration of heat treatment parameters to achieve desired properties. Welding procedures may need special attention, particularly in high-carbon grades. The selection of specific grades should match application requirements, taking into account the operating environment's corrosion challenges. Their continued development, particularly in improving corrosion resistance and toughness through nitrogen and nickel additions, ensures their ongoing relevance in modern engineering applications.