In metallurgy, stainless steel is a steel alloy with at least 10.5% chromium with or without other alloying elements and a maximum of 1.2% carbon by mass. Stainless steels, also known as inox steels or inox from French inoxydable (inoxidizable), are steel alloys that are very well known for their corrosion resistance, which increases with increasing chromium content. Corrosion resistance may also be enhanced by nickel and molybdenum additions. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 10.5% by weight, above which passivity can occur and below is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties.
PH Stainless Steel
PH stainless steels (precipitation-hardening) contain around 17% chromium and 4% nickel. These steels can develop very high strength through additions of aluminum, titanium, niobium, vanadium, and/or nitrogen, which form coherent intermetallic precipitates during a heat treatment process referred to as heat aging. As the coherent precipitates form throughout the microstructure, they strain the crystalline lattice and impede the movement of dislocations or defects in a crystal’s lattice. Since dislocations are often the dominant carriers of plasticity, this hardens the material. Precipitation-hardening stainless steels have high toughness, strength, and corrosion resistance. Precipitation-hardening stainless steels have been increasingly used for various applications in marine construction, aircraft and gas turbines, chemical industries, and nuclear power plants.
17-4PH Stainless Steel
For example, precipitation-hardened stainless steel 17-4 PH (AISI 630) has an initial microstructure of austenite or martensite. Austenitic grades are converted to martensitic grades through heat treatment (e.g., heat treatment at about 1040 °C followed by quenching) before precipitation hardening. Subsequent aging treatment at about 475 °C precipitates Nb and Cu-rich phases, increasing the strength to above 1000 MPa yield strength. In all heat treatments performed, the predominant microstructure is lath martensite. Unlike austenitic alloys, heat treatment strengthens PH steel to levels higher than martensitic alloys. Precipitation-hardening stainless steels are designated by the AISI 600-series. Of all the available stainless grades, they generally offer the greatest combination of high strength and excellent toughness and corrosion resistance. They are as corrosion-resistant as austenitic grades. Common uses are in the aerospace and some other high-technology industries.
In the nuclear industry, 17-4PH steel may be used in control rods. Control rods usually constitute cluster control rod assemblies (PWR) and are inserted into guide thimbles within a nuclear fuel assembly. The absorbing material (e.g., pellets of boron carbide) is protected by the cladding, usually made of stainless steel. The control element has some components that need high resistance to impacts when the safety function is activated, so the material of this component must have high mechanical strength and toughness. One of the materials specified for this application is PH stainless steel 17-4PH (UNS 17400).
Properties of PH Stainless Steel
Material properties are intensive properties, which means they are independent of the amount of mass and may vary from place to place within the system at any moment. Materials science involves studying materials’ structure and relating them to their properties (mechanical, electrical, etc.). Once materials scientist knows about this structure-property correlation, they can then go on to study the relative performance of a material in a given application. The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and how it has been processed into its final form.
Mechanical Properties of PH Stainless Steel
Materials are frequently chosen for various applications because they have desirable combinations of mechanical characteristics. For structural applications, material properties are crucial, and engineers must consider them.
Strength of PH Stainless Steel
In the mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. The strength of materials considers the relationship between the external loads applied to a material and the resulting deformation or change in material dimensions. The strength of a material is its ability to withstand this applied load without failure or plastic deformation.
Ultimate Tensile Strength
The ultimate tensile strength of precipitation hardening steel – 17-4PH stainless steel depends on the heat treatment process, but it is about 1000 MPa.
The ultimate tensile strength is the maximum on the engineering stress-strain curve. This corresponds to the maximum stress sustained by a structure in tension. Ultimate tensile strength is often shortened to “tensile strength” or “the ultimate.” If this stress is applied and maintained, a fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after achieving the ultimate strength. It is an intensive property; therefore, its value does not depend on the size of the test specimen. However, it depends on other factors, such as the specimen preparation, the presence or otherwise of surface defects, and the temperature of the test environment and material. Ultimate tensile strengths vary from 50 MPa for aluminum to as high as 3000 MPa for very high-strength steel.
The yield strength of precipitation hardening steels – 17-4PH stainless steel depends on heat treatment, but it is about 850 MPa.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically. In contrast, the yield point is where nonlinear (elastic + plastic) deformation begins. Before the yield point, the material will deform elastically and return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behavior termed a yield point phenomenon. Yield strengths vary from 35 MPa for low-strength aluminum to greater than 1400 MPa for high-strength steel.
Young’s Modulus of Elasticity
Young’s modulus of elasticity of precipitation hardening steel – 17-4PH stainless steel is 200 GPa.
Young’s modulus of elasticity is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to limiting stress, a body will be able to recover its dimensions on the removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position, and all the atoms are displaced the same amount and maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions, and no permanent deformation occurs. According to Hooke’s law, the stress is proportional to the strain (in the elastic region), and the slope is Young’s modulus. Young’s modulus is equal to the longitudinal stress divided by the strain.
The hardness of PH Stainless Steel
Brinell hardness of precipitation hardening Steels – 17-4PH stainless steel is approximately 353 MPa.
In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, abrasion, indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
Brinell hardness test is one of the indentation hardness tests developed for hardness testing. In Brinell tests, a hard, spherical indenter is forced under a specific load into the surface of the metal to be tested. The typical test uses a 10 mm (0.39 in) diameter hardened steel ball as an indenter with a 3,000 kgf (29.42 kN; 6,614 lbf) force. The load is maintained constant for a specified time (between 10 and 30 s). For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball.
The test provides numerical results to quantify the hardness of a material, which is expressed by the Brinell hardness number – HB. The Brinell hardness number is designated by the most commonly used test standards (ASTM E10-14 and ISO 6506–1:2005) as HBW (H from hardness, B from Brinell, and W from the material of the indenter, tungsten (wolfram) carbide). In former standards, HB or HBS were used to refer to measurements made with steel indenters.
The Brinell hardness number (HB) is the load divided by the surface area of the indentation. The diameter of the impression is measured with a microscope with a superimposed scale. The Brinell hardness number is computed from the equation:
There are various test methods in common use (e.g., Brinell, Knoop, Vickers, and Rockwell). Some tables correlate the hardness numbers from the different test methods where correlation is applicable. In all scales, a high hardness number represents a hard metal.
Thermal Properties of PH Stainless Steel
Thermal properties of materials refer to the response of materials to changes in their temperature and the application of heat. As a solid absorbs energy in the form of heat, its temperature rises, and its dimensions increase. But different materials react to the application of heat differently.
Melting Point of PH Stainless Steel
The melting point of precipitation hardening steels – 17-4PH stainless steel is around 1450°C.
In general, melting is a phase change of a substance from the solid to the liquid phase. The melting point of a substance is the temperature at which this phase change occurs. The melting point also defines a condition where the solid and liquid can exist in equilibrium.
Thermal Conductivity of PH Stainless Steel
The thermal conductivity of precipitation hardening steels – 17-4PH stainless steel is 18 W/(m. K).
The heat transfer characteristics of solid material are measured by a property called the thermal conductivity, k (or λ), measured in W/m.K. It measures a substance’s ability to transfer heat through a material by conduction. Note that Fourier’s law applies to all matter, regardless of its state (solid, liquid, or gas). Therefore, it is also defined for liquids and gases.
The thermal conductivity of most liquids and solids varies with temperature, and for vapors, it also depends upon pressure. In general:
Most materials are nearly homogeneous. Therefore we can usually write k = k (T). Similar definitions are associated with thermal conductivities in the y- and z-directions (ky, kz), but for an isotropic material, the thermal conductivity is independent of the direction of transfer, kx = ky = kz = k.