Mechanical Properties of Stainless Steels

Stainless Steel Corrosion

There are five main types of stainless steel: ferritic, martensitic, austenitic, precipitation hardening and duplex. The ferritic and martensitic grades are so named because of their crystal structures. Both are iron-chromium-based alloys and were the type of stainless steel first developed in the early 1900’s. The ferritic and martensitic stainless steels are magnetic. The martensitic stainless steels can be hardened by a heat treatment similar to that used to harden ordinary steel, namely, heating to a high temperature, quenching, then reheating to an intermediate temperature (tempering) to achieve the desired balance of hardness and ductility.

Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at atmospheric and elevated temperatures, and are produced to cover a wide range of mechanical and physical properties for particular applications. Along with iron and chromium, all stainless steels contain some carbon. It is difficult to get much less than about 0.03 % and sometimes carbon is deliberately added up to 1.00% or more. The more carbon there is, the more chromium must be used, because carbon can take from the alloy about seventeen times its own weight of chromium to form carbides. Chromium carbide is of little use for resisting corrosion. The carbon, of course, is added for the same purpose as in ordinary steels to make the alloy stronger.

Stainless steels are mainly used in wet environments. With increasing chromium and molybdenum contents, the steels become increasingly resistant to aggressive solutions. The higher nickel content reduces the risk of SCC. Austenitic steels are more or less resistant to general corrosion, crevice corrosion and pitting, depending on the quantity of alloying elements. Resistance to pitting and crevice corrosion is very important if the steel is to be used in chloride containing environments. Resistance to pitting and crevice corrosion typically increases with increasing contents of chromium, molybdenum and nitrogen.

Corrosion resistance of stainless steels is a function not only of composition, but also of heat treatment, surface condition, and fabrication procedures, all of which may change the thermodynamic activity of the surface and thus dramatically affect the corrosion resistance. It is not necessary to chemically treat stainless steels to achieve passivity. The passive film forms spontaneously in the presence of oxygen. Most frequently, when steels are treated to improve passivity (passivation treatment), surface contaminants are removed by pickling to allow the passive film to reform in air, which it does almost immediately. Most of the ferritic and martensitic stainless steels have limited corrosion resistance in marine environments, but some of the newly developed ferritic grade s (often called “superferritics”) have excellent marine corrosion resistance and are widely used in applications such as tubes for power plant condensers.

Pickling and Passivation

Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous chromium-oxide, passive film. Pickling and passivation are both acid treatments and neither will remove grease or oil. If the fabrication is dirty, it may be necessary to use a detergent or alkaline clean before pickling or passivation. (reference)

Stainless Steel Weld Decay

This type of intergranular corrosion can occur in the heat-affected zone of welded components and also in cast components of stainless steel due to precipitation, during cooling, of chromium carbides at the grain boundaries (and hence loss of chromium in the immediately-adjacent zone). The local loss in corrosion resistance arises because the chromium is crucial in promoting the formation of a Cr-rich passive film on the surface of stainless steels. The susceptibility to weld decay can be counteracted by carrying out a suitable post-weld heat treatment to restore a uniform composition at the grain boundaries but this is clearly often not a practicable proposition. Consequently the usual strategy in combating weld decay is by the choice of stainless steel with either of the two following features:

1. specification of a stainless steel containing a small amount of either titanium or niobium; which have a higher affinity than does chromium for carbon: hence carbides of these elements tend to form instead of chromium carbides, thus avoiding the Cr-depletion problem: such steels are usually termed “stabilised stainless steels”
2. specification of a stainless steel with low carbon content (< 0.03%); this will clearly decrease the likelihood of carbide formation in the steel. Such low-carbon grades of stainless steel are often designated by a “L” in their code; for instance the “316” grade of steel (18%Cr/10Ni/2.5Mo) is designated as “316L” when its carbon content has been limited in this way.

Stress corrosion cracking (SCC)

Austenitic stainless steels suffer from stress corrosion cracking in hot solutions containing chloride. A high chloride concentration is required, although relatively small amounts of chloride are sufficient at heated surfaces, where chloride concentration can occur, or where chloride is concentrated by pitting or crevice corrosion, and problems can be experienced in tap water.

The temperature usually needs to be above 70°C, although SCC can occur at lower temperatures in some situations, notably more acid solutions. The cracking continues at low stresses and commonly occurs as a result of residual stresses from welding or fabrication. The cracking is normally transgranular, although it may switch to an intergranular path as a result of sensitization of the steel.

Stainless Steel Rouging

Rouging is a thin film, usually reddish-brown or golden in color, of iron oxide or hydroxide, typically on stainless steels. The contrast between this film and shiny metal accentuates this aesthetics problem. The rouge film typically wipes off easily with a light cloth (Figure 1), but it reforms while the process fluid is in contact with the stainless steel. This problem is most chronic in the pharmaceutical industry on the interior surfaces of high purity water (i.e., water for injection, WFI) distillation units, storage tanks, distribution systems (piping, valves, pump housings, fittings, etc.) and process vessels. (reference)

Many pages of the Corrosion Doctors' Web site discuss specific issues related to the corrosion behavior of steels. The following are references to some of these pages:

Additional Resources:

1. Introduction

   Often materials are subject to forces (loads) when they are used. Mechanical engineers calculate those forces and material scientists how materials deform (elongate, compress, twist) or break as a function of applied load, time, temperature, and other conditions.

   Materials scientists learn about these mechanical properties by testing materials. Results from the tests depend on the size and shape of material to be tested (specimen), how it is held, and the way of performing the test. That is why we use common procedures, or standards, which are published by the ASTM.

2. Concepts of Stress and Strain

   To compare specimens of different sizes, the load is calculated per unit area, also called normalization to the area. Force divided by area is called stress. In tension and compression tests, the relevant area is that perpendicular to the force. In shear or torsion tests, the area is perpendicular to the axis of rotation.

   s = F/A₀ tensile or compressive stress

   t = F/A₀ shear stress

   The unit is the Megapascal = 10⁶ Newtons/m².

   There is a change in dimensions, or deformation elongation, DL as a result of a tensile or compressive stress. To enable comparison with specimens of different length, the elongation is also normalized, this time to the length L. This is called strain, e.

   e = DL/L

   The change in dimensions is the reason we use A₀ to indicate the initial area since it changes during deformation. One could divide force by the actual area, this is called true stress (see Sec. 6.7).

   For torsional or shear stresses, the deformation is the angle of twist, q (Fig. 6.1) and the shear strain is given by:

   g = tg q

3. Stress—Strain Behavior

   Elastic deformation. When the stress is removed, the material returns to the dimension it had before the load was applied. Valid for small strains (except the case of rubbers).

   Deformation is reversible, non permanent

   Plastic deformation. When the stress is removed, the material does not return to its previous dimension but there is a permanent, irreversible deformation.

   In tensile tests, if the deformation is elastic, the stress-strain relationship is called Hooke's law:

   s = E e

   That is, E is the slope of the stress-strain curve. E is Young's modulus or modulus of elasticity. In some cases, the relationship is not linear so that E can be defined alternatively as the local slope:

   E = ds/de

   Shear stresses produce strains according to:

   t = G g

   where G is the shear modulus.

   Elastic moduli measure the stiffness of the material. They are related to the second derivative of the interatomic potential, or the first derivative of the force vs. internuclear distance (Fig. 6.6). By examining these curves we can tell which material has a higher modulus. Due to thermal vibrations the elastic modulus decreases with temperature. E is large for ceramics (stronger ionic bond) and small for polymers (weak covalent bond). Since the interatomic distances depend on direction in the crystal, E depends on direction (i.e., it is anisotropic) for single crystals. For randomly oriented policrystals, E is isotropic.

4. Anelasticity

   Here the behavior is elastic but not the stress-strain curve is not immediately reversible. It takes a while for the strain to return to zero. The effect is normally small for metals but can be significant for polymers.

5. Elastic Properties of Materials

   Materials subject to tension shrink laterally. Those subject to compression, bulge. The ratio of lateral and axial strains is called the Poisson's ratio n.

   n = e_lateral/e_axial

   The elastic modulus, shear modulus and Poisson's ratio are related by E = 2G(1+n)

6. Tensile Properties

   Yield point. If the stress is too large, the strain deviates from being proportional to the stress. The point at which this happens is the yield point because there the material yields, deforming permanently (plastically). Yield stress. Hooke's law is not valid beyond the yield point. The stress at the yield point is called yield stress, and is an important measure of the mechanical properties of materials. In practice, the yield stress is chosen as that causing a permanent strain of 0.002 (strain offset, Fig. 6.9.)

   The yield stress measures the resistance to plastic deformation.

   The reason for plastic deformation, in normal materials, is not that the atomic bond is stretched beyond repair, but the motion of dislocations, which involves breaking and reforming bonds.

   Plastic deformation is caused by the motion of dislocations.

   Tensile strength. When stress continues in the plastic regime, the stress-strain passes through a maximum, called the tensile strength (s_TS) , and then falls as the material starts to develop a neck and it finally breaks at the fracture point (Fig. 6.10).

   Note that it is called strength, not stress, but the units are the same, MPa.

   For structural applications, the yield stress is usually a more important property than the tensile strength, since once the it is passed, the structure has deformed beyond acceptable limits.

   Ductility. The ability to deform before braking. It is the opposite of brittleness. Ductility can be given either as percent maximum elongation e_max or maximum area reduction.

   %EL = e_max x 100 %    

   %AR = (A₀ - A_f)/A₀

   These are measured after fracture (repositioning the two pieces back together).Resilience. Capacity to absorb energy elastically. The energy per unit volume is the

   area under the strain-stress curve in the elastic region. Toughness. Ability to absorb energy up to fracture. The energy per unit volume is the total area under the strain-stress curve. It is measured by an impact test (Ch. 8).

7. True Stress and Strain

   When one applies a constant tensile force the material will break after reaching the tensile strength. The material starts necking (the transverse area decreases) but the stress cannot increase beyond s_TS. The ratio of the force to the initial area, what we normally do, is called the engineering stress. If the ratio is to the actual area (that changes with stress) one obtains the true stress.

8. Elastic Recovery During Plastic Deformation

   If a material is taken beyond the yield point (it is deformed plastically) and the stress is then released, the material ends up with a permanent strain. If the stress is reapplied, the material again responds elastically at the beginning up to a new yield point that is higher than the original yield point (strain hardening, Ch. 7.10). The amount of elastic strain that it will take before reaching the yield point is called elastic strain recovery (Fig. 6. 16).

9. Compressive, Shear, and Torsional Deformation

   Compressive and shear stresses give similar behavior to tensile stresses, but in the case of compressive stresses there is no maximum in the s-e curve, since no necking occurs. 

10. Hardness

    Hardness is the resistance to plastic deformation (e.g., a local dent or scratch). Thus, it is a measure of plastic deformation, as is the tensile strength, so they are well correlated. Historically, it was measured on an empirically scale, determined by the ability of a material to scratch another, diamond being the hardest and talc the softer. Now we use standard tests, where a ball, or point is pressed into a material and the size of the dent is measured. There are a few different hardness tests: Rockwell, Brinell, Vickers, etc. They are popular because they are easy and non-destructive (except for the small dent).

11. Variability of Material Properties

    Tests do not produce exactly the same result because of variations in the test equipment, procedures, operator bias, specimen fabrication, etc. But, even if all those parameters are controlled within strict limits, a variation remains in the materials, due to uncontrolled variations during fabrication, non homogenous composition and structure, etc. The measured mechanical properties will show scatter, which is often distributed in a Gaussian curve (bell-shaped), that is characterized by the mean value and the standard deviation (width).

12. Design/Safety Factors

    To take into account variability of properties, designers use, instead of an average value of, say, the tensile strength, the probability that the yield strength is above the minimum value tolerable. This leads to the use of a safety factor N > 1 (typ. 1.2 - 4). Thus, a working value for the tensile strength would be s_W = s_TS / N.