Heat Treatment of Steels

Heat Treatment of Steels

Different Heat Treatment MethodsWhat Is Annealing?

Annealing is the process of heating the steel to a particular temperature in the austenite region and cooling down the steel very slowly. There are many derivatives of the annealing process, but generally the process is a slow cool process.Another derivative of the annealing process is known as sub-critical anneal. This process involves soaking at a temperature below the lower transformation line, in the region of 1,200˚F to 1,300˚F, until the steel has equalized across its cross-section in temperature, followed by a slow cool. Slow cooling can mean a cooling rate between 5˚F per hour up to 50˚F per hour.As can be imagined, the cooling period can be a considerable amount of time. It should be noted that the nickel alloyed steels and the A series tool steels should be cooled very slowly, as nickel will cause an air-hardening effect.

Other Types of Annealing: • Bright Anneal. This method is a method of annealing which uses a protective

atmosphere to prevent the steel surface from oxidation. • Process Anneal. This procedure is done at a temperature close to the lower critical

line on the iron carbon diagram. Sometimes confused with sub-critical annealing, it is used when considerable cold working is to follow.

• Recrystallization Anneal. Once again, this is a process often mistaken for subcritical annealing. It is used after cold working to produce a specific grain structure.

• Sub-Critical Anneal. This method is used on cold-worked steel and is carried out below the lower critical line on the iron carbon equilibrium diagram. It is sometimes applied to tool steels that have been over tempered and require annealing before hardening and tempering.

• Spheroidize Anneal. This process is a controlled heating and cooling procedure to produce spheroidal or globular cementite particles. It is usually applied on high carbon steels for good machining characteristics such as high alloy steels and tool steels.

• Isothermal Annealing. The process temperature of this procedure is determined by knowledge of the steel's carbon content. The steel is then taken to that temperature and cooled down to a holding temperature that allows the steel to transform isothermally.

• Full Anneal. This is a process that involves raising the steel's temperature up to the sustenite region followed by a slow cool.

What Is Normalizing?Normalizing is a process that makes the grain size normal. This process is

usually carried out after forging, extrusion, drawing or heavy bending operations.When steel is heated to elevated temperatures to complete the above

operations, the grain of the steel will grow. In other words, the steel experiences a phenomenon called "grain growth."

This leaves the steel with a very coarse and erratic grain structure. Furthermore, when the steel is mechanically deformed by the aforementioned operations, the grain becomes elongated.

There are mechanical property changes that take place as a result of normalizing - inasmuch as the normalized steel is soft, but not as soft as a fully annealed steel. Its grain structure is not as coarse as an annealed steel, simply because the cooling rate is faster than that of annealing. Usually the steel is cooled in still air and free from air drafts. The process temperature is virtually the same as for annealing, but the results are different due to the cooling rate.

• The process is designed to: • Give improved machining characteristics. • Ensure a homogenous structure. • Reduce residual stresses from rolling and forging. • Reduce the risk of "banding." • Help to give a more even response to the steel when hardening.

What Is Stress Relieving?

Stress relieving is an intermediate heat treatment procedure to reduce induced residual stresses as a result of machining, fabrication and welding. The application of heat to the steel during its machining or fabrication will assist in removing residual stresses that will, unless addressed during the manufacturing by stress relieving, manifest themselves at the final heat treatment procedure.

It is a relatively low temperature operation that is done in the ferrite region, which means that there is no phase change in the steel, only the reduction of residual stresses. The temperature region is usually between 800xF to 1,300xF. However, the higher that one goes in temperature, the greater the risk of surface oxidation there is. It is generally better to keep to the lower temperatures, particularly if the steel is a "pre-hard" steel. The hardness will be reduced if the stress relieve temperature exceeds the tempering temperature of the steel.

There is a general rule of thumb for time at temperature. It must be stated that the time is taken when the part is at temperature, not when the furnace is at temperature. The time at temperature for the processes of full anneal (not spheroidize anneal), normalize and stress relieve is 60 minutes at part temperature per one-inch of the maximum cross-sectional area.

Precipitation hardeningSome metals are classified as precipitation hardening metals.

When a precipitation hardening alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age "naturally" meaning that the precipitates form at room temperature, or they may age "artificially" when precipitates only form at elevated temperatures. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations - assembly of rivets, for example, may be easier with a softer part.

Examples of precipitation hardening alloys include 2000 series, 6000 series, and 7000 series aluminum alloy, as well as some superalloys and some stainless steels.

Selective hardeningSome techniques allow different areas of a single object to receive

different heat treatments. This is called differential hardening. It is common in high quality knives and swords. The Chinese jian is one of the earliest known examples of this, and the Japanese katana the most widely known. The Nepalese Khukuri is another example.

Case hardeningCase hardening is a process in which an alloying element, most

commonly carbon or nitrogen, diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.

Laser surface engineering is a surface treatment with high versatility, selectivity and novel properties. Since the cooling rate is very high in laser treatment, metastable even metallic glass can be obtained by this method.

Through hardeningOnly hardness is listed for through hardening. It is usually in the

form of HRC with at least a five point range.

The Fe-C Phase Diagram

The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig 1). Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or metastable equilibrium) for different combinations of carbon concentration and temperature.

We distinguish at the low-carbon end ferrite (α-iron),which can at most dissolve 0.028% C, at 727°C (1341°F) and austenite -iron, which can dissolve 2.11 wt% C at 1148°C (2098°F). At the carbon-rich side we find cementite (Fe3C). Of less interest, except for highly alloyed steels, is the δ-ferrite existing at the highest temperatures.

Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + δ-ferrite.

In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names:

• A1, the so-called eutectoid temperature, which is the minimum temperature for austenite

• A3, the lower-temperature boundary of the austenite region at low carbon contents, that is, the γ/γ + α boundary

• Acm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe3C boundary

The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content (0.77 wt% C). The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that degenerates into cementite particles dispersed with a ferrite matrix after extended holding close to A1.

The Fe-C diagram in Fig 1 is of experimental origin. The knowledge of the thermodynamic principles and modern thermodynamic data now permits very accurate calculations of this diagram. This is particularly useful when phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to develop.

If alloying elements are added to the iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries and the eutectoid composition are changed. It suffices here to mention that

1. all important alloying elements decrease the eutectoid carbon content,

2. the austenite-stabilizing elements manganese and nickel decrease A, and

3. the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A1.

Reasons for Heat Treatment

Making Parts StrongerTo fully understand the advantages of heat-treating

processes to manufacturing it is important to first understand a fundamental principal of metals – structure. Elements such as Aluminum (Al), Chromium (Cr), Copper (Cu), Iron (Fe), Molybdenum (Mo), Nickel (Ni) and Silicon (Si) are a few examples of metals having these crystal structures.

If a load is applied to a metal it will cause the metal to deform first by elastic deformation and then, if enough force is applied, by plastic deformation. The strength of the electromagnetic force between atoms determines the yield strength as well as the ultimate tensile strength of the material.

Alloying elements help make metals stronger and more resistant to deformation by strengthening their crystal structures.

Crystal structureA unique arrangement of atoms in a metal. A crystal

structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice. Motifs are located upon the points of a lattice, which is an array of points repeating periodically in three dimensions. The points can be thought of as forming identical tiny boxes called unit cells that fill the space of the lattice. The lengths of the edges of a unit cell and the angles between them are called the lattice parameters. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure and optical properties.

Elastic deformationThe type of deformation that is reversible. Once the

forces are no longer applied, the object returns to its original shape

Ultimate tensile strengthThe maximum stress a material can withstand when subjected to tension (as opposed to compression or shearing). It is the maximum value on the stress-strain curve at which a material breaks or permanently deforms. Tensile strength is an intensive property and, consequently, does not depend on the size of the test specimen. However, it is dependent on the preparation of the specimen and the temperature of the test environment and material. Tensile strength, along with elastic modulus and corrosion resistance, is an important parameter of engineering materials that are used in structures and mechanical devices.

Yield strength (or yield point)The stress at which a material begins to deform plastically. Prior to the yield point, the material will deform elastically and will 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.

Diffusion TreatmentIn the pack diffusion coating of chromium into the surface of a superalloy,

the formation of undesirable oxide inclusion is reduced when the diffusion coating pack contains at least about 3% Ni3 Al. Also the formation of alpha-chromium is reduced when the pack diffusion is carried out in a retort effectively not over five inches in height. Pack aluminizing in the presence of chromium makes a very effective aluminum- and chromium-containing top coating over platinum plated or platinum coated nickel-base superalloys. Aluminized nickel can also have its aluminum attacked and at least partially removed with aqueous caustic to leave a very highly active catalytic surface. Pack diffusion can also be arranged to simultaneously provide different coatings in different locations by using different pack compositions in those locations. An aluminizing pack containing a large amount of chromium provides a thinner aluminized case than an aluminizing pack containing less chromium, or less chromium and some silicon. A cobalt-chromium pack deposits essentially a chromized case when energized with a chloride, but deposits large amounts of cobalt along with chromium when energized with an iodide. Even more chromium with large amounts of cobalt are deposited when the energizer is a mixture of iodide and chloride. Depletion of diffusible material from workpieces heated in a powder-pack can also be readily controlled by adjusting the pack composition, and such depletion from cobalt-base superalloys very simply provides a surface on which aluminizing produces a highly impact-resistant coating.