General Corrosion Resistance

Stainless steels are characterized by a very good aqueous corrosion resistance and by a very good resistance to oxidation at high temperatures. All stainless steels contain at least 11% Cr. Many contain nickel as well. For the aqueous corrosion resistance, the steels must contain a minimum of 11.5% chromium, which makes them passive in oxidizing solutions. Even more chromium is required for passivity in nonoxidizing solutions. Unless the chromium content is sufficient for passivity, the corrosion resistance of stainless steels is similar to steels without any chromium. Table 19.1 is a galvanic series of alloys. It shows that stainless steels may occupy two positions corresponding to the active and passive conditions.

There are five major types of stainless steels: ferritic, martensitic, austenitic, duplex, and precipitation hardenable.

The influence of alloying elements on the rate of pearlite formation influences whether martensite will be formed when austenite is quenched because martensite can form only from austenite. If the formation of pearlite is delayed, more austenite will be available at the Ms temperature to transform to martensite. The term hardenability is used to describe this effect. We say that alloying elements increase the hardenability of steel, making it possible to harden them to greater depths.

Jominy End-Quench Test

Hardenability may be quantitatively described several ways. One of the simplest is the Jominy end-quench test in which a 4-in.-long, one-inch-diameter bar of the steel is austenitized and then placed in a fixture and cooled from one end with a specified water spray (Figure 13.1). The hardness is then measured as a function of distance from the quenched end. Figures 13.2 and 13.3 show the resulting curves for several steels.

General

Annealing is the heating of metal after it has been cold worked to soften it. Most of the energy expended in cold work is released as heat during the deformation. However, a small percent of the mechanical work is stored by dislocations and vacancies. The stored energy is the driving force for the changes during annealing. There are three stages of annealing. In order of increasing time and temperature, they are as follows:

1. Recovery – often a small drop in hardness and rearrangement of dislocations to form subgrains. Otherwise, overall grain shape and orientation remain unchanged. Residual stresses are relieved.

2. Recrystallization – replacement of cold-worked grains with new ones. There are new orientations, a new grain size, and a new grain shape, but not necessarily equiaxed. Recrystallization causes the major hardness decrease.

3. Grain growth – growth of recrystallized grains at the expense of other recrystallized grains.

Recovery

The energy release during recovery is largely due to annealing out of point defects and rearrangement of dislocations. Most of the increase of electrical resistivity during cold work is attributable to vacancies. These anneal out during recovery, so that the electrical resistivity drops (Figure 6.1) before any major hardness changes occur. During recovery, residual stresses are relieved, and this decreases the energy stored as elastic strains. The changes during recovery cause no changes in microstructure that would be observable under a light microscope. Figure 6.2 shows the energy release and the changes of resisitivity and hardness with increasing annealing temperatures.

Native Iron

The only sources of iron available to early humans were meteoric iron and native (telluric) iron. Both were scarce. Most meteorites are nonmetallic; only about 6% are iron, and these contain about 7 to 15% nickel. In 1808, William Thomson sectioned and etched a meteorite, noting the remarkable patterns. Although he published his findings in 1808, they attracted little interest. Also in 1808, an Austrian, Alois von Widmannstätten, also etched a meteorite and observed the structure that is now known by his name. In 1820, he and Carl von Schreibers published a book on meteorites, which contained a print from a heavily etched meteorite (Figure 2.1). Native iron is even scarcer, being limited to small particles in western Greenland. Archeological finds of iron with considerable amounts of nickel suggest that they were made from meteorites.

The first production of iron dates back to at least 2000 bc in India and Sri Lanka. By 1200 bc, production of iron was widespread in China and the Near East. The most common iron ores are hematite (Fe2O3) and magnetite (Fe3O4). Smelting of iron involved heating iron ore (oxides of iron) with charcoal. The reaction of iron oxide with carbon produced carbon monoxide and carbon dioxide. The air was supplied by either a natural draft or some means of blowing. Early furnaces were of various types. An open-pit furnace is shown in Figure 2.2. The carbon content of iron produced in pit furnaces was usually low because of the low temperatures achieved and resulted in semisolid sponge.

Slip and Twinning Systems

The direction of slip in iron as in all body-centered cubic (bcc) metals is <111>. This is the direction of the shortest repeat distance in the bcc lattice. Slip can occur on any plane containing a <111> direction. The critical stress for slip is lowest for the {110} plane and highest for the {112}, the difference increasing as the temperature decreases. There is also an asymmetry to slip on {112}. The shear stress for slip is least when the direction is the same as for twinning and highest in the anti-twinning direction. See Figure 7.1.

For iron and all bcc metals, the twinning direction is <111>, and the twinning plane is {112}. The twinning shear strain is 1/√2 = 0.707. Twins, called Neuman bands, are very narrow and occur only during deformation at low temperatures or high strain rates. At 4.2 K, the critical shear stress for twinning is about 520 MPa. Twinning normally contributes little to the overall deformation.

Yielding and Lüders Bands

A tensile stress-strain curve of an annealed low-carbon steel is shown in Figure 11.1. Loading is elastic until an upper yield stress is reached (Point A). Then the load suddenly drops to a lower yield stress (Point B). The reason for this phenomenon is that during annealing, interstitially dissolved carbon and nitrogen atoms tend to diffuse to edge dislocations. Because they partially relieve the stress field around the dislocations (Figure 11.2), they lower the energy. A higher stress is required to break the dislocations free from the interstitial atoms than to move them once they have broken free. Continued elongation after initial yielding occurs by the propagation of the yielded region at this lower yield stress until the entire gauge section has yielded (point C). During this period of yielding, there is a sharp boundary or Lüders band between the region that has yielded and the region that has not. Behind this boundary, all of the material has suffered the same strain. The Lüders strain or yield point elongation in low-carbon steels is typically 1 to 5%. Strain hardening starts only after the Lüders band has traversed the entire gauge section. Finally, at some point, D, the specimen undergoes necking. Figure 11.3 shows Lüders bands that have partially traversed the gauge section.

Strain Aging

If the specimen were unloaded at some point, F, and immediately reloaded, the stress-strain curve would follow the original curve. However, if the unloaded specimen were allowed to strain age, a new yield point would develop (Aʹ). Strain aging would occur if the time was long enough and the temperature were high enough to allow interstitial carbon and nitrogen atoms to diffuse to the dislocations. Figure 11.4 shows that the amount of strain aging increases with the time at room temperature between unloading and reloading.

Hadfield Austenitic Manganese Steel

Manganese is a powerful austenite stabilizer as indicated by the Fe-Mn phase diagram (see Figure 8.2). Hadfield manganese steels containing 10 to 14% Mn and 1 to 1.4% C are austenitic at all temperatures. They are extremely wear resistant and are used in ore grinding and for teeth on earth-moving equipment. These steels work harden rapidly and as a consequence are difficult to machine. Parts are almost always cast to final shape.

Maraging Steels

Maraging steels develop high hardness by precipitation hardening of a very-low-carbon martensite. They contain about 18% Ni, 8 to 12% Co, and 4% Mo with very low carbon (<0.03%), as well as titanium (0.20 to 1.80%), and aluminum (0.10 to 0.15%). Their primary use is in tools and dies. Because the carbon content is so low, the martensite, which is formed by austenitizing at 850°C and cooling, is soft enough to be machined. Finished tooling can then be hardened by aging at 480°C for 3 hours. Whereas tools made from conventional tool steels have to be austenitized, quenched, and tempered after machining, maraging steels need only be heated to a moderate temperature to age and can be furnace cooled. This avoids oxidation, distortion, and cracking that often occurs during conventional heat treatment. The main disadvantage is the high cost that results from the high Ni, Mo, and Co contents.

Microstructures of Carbon Steels

Steels are iron-based alloys. The most common are carbon steels, which may contain up to 1.5% carbon. Cast irons typically contain between 2.5 and 4% carbon. Figure 4.1 is the phase diagram showing the metastable equilibrium between iron and iron carbide. Below 912°C, pure iron has a body-centered cubic (bcc) crystal structure and is called ferrite, which is designated by the symbol α. Between 912 and 1400°C, the crystal structure is face-centered cubic (fcc). This phase, called austenite, is designated by the symbol γ. Between 1400°C and the melting point, iron is again bcc. This phase is called δ-ferrite, but it is really no different from α-ferrite. The maximum solubility of carbon in α (bcc iron) iron is 0.02% C and in γ (fcc iron) is about 2%. Iron carbide, Fe3C, is called cementite and has a composition of 6.67% C. The structure developed by the eutectoid reaction, γ → α + Fe3C at 727°C, consists of alternating platelets of ferrite and carbide (Figure 4.2) and is called pearlite.

Steels containing less than 0.77% C are called hypoeutectoid, and those with more than 0.77% C are called hypereutectoid. The microstructures of medium-carbon steels (0.2 to 0.7% C) depend on how rapidly they are cooled from the austenitic temperature. If the cooling is very slow (furnace cooling), the proeutectoid ferrite will form in the austenite grain boundaries, surrounding regions of austenite that subsequently transforms to pearlite, as shown in Figure 4.3.

Tempering

Martensite is brittle. To make it tougher, it is normally heated to temper it. Tempering involves a complex series of reactions that gradually break down martensite. What happens is usually described in stages. The first stage occurs at the lowest temperature (shortest time) and involves transformation of retained austenite. In the second stage, carbon is redistributed within the martensite to dislocations. Generally, stress relief occurs during this stage. Precipitation of ɛ carbide (Fe2.4C) and η carbide (Fe2C) from the martensite comprises the third stage. This precipitation lowers the carbon content of the martensite. In stage four, remaining austenite decomposes to cementite (Fe3C) and ferrite. Finally, in stage five, the transition carbides and low-carbon martensite form more ferrite and cementite. These reactions overlap.

There is a gradual loss of hardness throughout tempering (except stage one) at increasing temperatures. This is shown as a function of the carbon content, as shown in Figure 14.1. Figure 14.2 shows the effect of tempering temperature on the properties of 4340 steel. The amount of tempering depends on the carbon content (Figure 14.3) as well as time and temperature (Figure 13.4), although the effect of time is much less than that of temperature.