Thermal processing of metals and alloys

 Thermal processing of metals and alloys 

    Apart from mechanical processing, metals are very often subjected to thermal processing for various reasons, like: to refine grain structure/size, to minimize residual stresses, to impart phase changes, to develop special phases over external surfaces, etc. Metals and alloys develop requisite properties by thermal processing either through grain refinement of phase changes. Thermal processing is also known as heat treatment. Heat treatment originated as an ancient art in man’s attempts to improve the performance of materials in their practical applications. In present day metallurgical practice, heat treatment has become very important for obvious reasons.
There has been tremendous progress over centuries in the systematic understanding of materials structure and structure-property relationships that eliminated the empiricism in thermal processing. Properly designed and implemented thermal processing can result in optimum modifications in the composition and distribution of phases, corresponding changes in physical, chemical and mechanical properties at substantial levels. However, most of the thermal processes are aimed to improving mechanical characteristics of materials. Thus it is possible to extend the service performance of materials considerably within constraints of available resources. All metals can be subjected to thermal processing. But the effect of it may differ from one metal to another. Metals are subjected to heat treatment for one or more of the following purposes: improvement in ductility; relieving internal stresses; grain size refinement; increase of strength; improvement in machinability, toughness; etc. Heat treatment of materials involves number of factors – temperature up to which material is heated, length of time that the material is held at the elevated temperature, rate of cooling, and the surrounding atmosphere under the thermal treatment. All these factors depend on material, pre-processing of the material’s chemical composition, size and shape of the object, final properties desired, material’s melting point/liquidus, etc. Thermal processes may be broadly classified into two categories based on cooling rates from elevated temperatures – annealing and quenching & tempering. Annealing involved cooling the material from elevated temperatures slowly, while quenching means very fast cooling of the material using cooling medium like water/oil bath. Quenching is done to retain the phases of elevated temperatures at room temperature.

Annealing processes in

   The term annealing was used by craftsmen who discovered the beneficial effects of heating the material at elevated temperatures followed by slow cooling of it to room temperature. Annealing can be defined as a heat treatment process in which the material is taken to a high temperature, kept there for some time and then cooled. High temperatures allow diffusion processes to occur fast. The time at the high temperature (soaking time) must be long enough to allow the desired transformation to occur. Cooling is done slowly to avoid the distortion (warping) of the metal piece, or even cracking, caused by stresses induced by differential contraction due to thermal inhomogeneities. Benefits of annealing are:

• relieve stresses 
•  increase softness, ductility and toughness
• produce a specific microstructure 

         Depending on the specific purpose, annealing is classified into various types: process annealing, stress relief, full annealing and normalizing. Process annealing is primarily applied to cold worked metals to negate the effects of cold work. During this heat treatment, material becomes soft and thus its ductility will be increased considerably. It is commonly sandwiched between two cold work operations. During this, recovery and recrystallization are allowed whereas grain growth was restricted. Stress relief operation removes the stresses that might have been generated during plastic deformation, non-uniform cooling, or phase transformation. Unless removed, these stresses may cause distortion of components. Temperature used is normally low such that effects resulting from cold working are not affected. Full annealing is normally used for products that are to be machined subsequently, such as transmission gear blanks. After heating and keeping at an elevated temperature, components are cooled in furnace to effect very slow cooling rates. Typically, the product receives additional heat treatments after machining to restore hardness and strength. Normalizing is used to refine the grains and produce a more uniform and desirable size distribution. It involves heating the component to attain single phase (e.g.: austenite in steels), then cooling in open air atmosphere. 

Quenching and Tempering processes 

       Quenching is heat treatment process where material is cooled at a rapid rate from elevated temperature to produce Martensite phase. This process is also known as hardening. Rapid cooling rates are accomplished by immersing the components in a quench bath that usually contains quench media in form of either water or oil, accompanied by stirring mechanism. Quenching process is almost always followed by tempering heat treatment. Tempering is the process of heating martensitic steel at a temperature below the eutectoid transformation temperature to make it softer and more ductile. During the tempering process, Martensite transforms to a structure containing iron carbide particles in a matrix of ferrite. Martempering is a modified quenching procedure used to minimize distortion and cracking that may develop during uneven cooling of the heat-treated material. It involves cooling the austenized steel to temperature just above Ms temperature, holding it there until temperature is uniform, followed by cooling at a moderate rate to room temperature before austenite-to-bainite transformation begins. The final structure of martempered steel is tempered Martensite. Austempering is different from martempering in the sense that it involves austenite-tobainite transformation. Thus, the structure of austempered steel is bainite. Advantages of austempering are – improved ductility; decreased distortion and disadvantages are – need for special molten bath; process can be applied to limited number of steels. 

Case Hardening 

        In case hardening, the surface of the steel is made hard and wear resistant, but the core remains soft and tough. Such a combination of properties is desired in applications such as gears. 

 Induction hardening

       Here, an alternating current of high frequency passes through an induction coil enclosing the steel part to be heat treated. The induced emf heats the steel. The depth up to which the heat penetrates and raises the temperature above the elevated temperature is inversely proportional to the square root of the ac frequency. In induction hardening, the heating time is usually a few seconds. Immediately after heating, water jets are activated to quench the surface. Martensite is produced at the surface, making it hard and wear resistant. The microstructure of the core remains unaltered. Induction hardening is suitable for mass production of articles of uniform cross-section.  

Flame hardening 

     For large work pieces and complicated cross-sections induction heating is not easy to apply. In such cases, flame hardening is done by means of an oxyacetylene torch. Heating should be done rapidly by the torch and the surface quenched, before appreciable heat transfer to the core occurs 

 Laser hardening 

      In this case, a laser beam can be used for surface hardening. As laser beams are of high intensity, a lens is used to reduce the intensity by producing a defocused spot of size ranging from 0.5 to 25 mm. Proper control of energy input is necessary to avoid melting. Laser hardening has the advantage of precise control over the area to be hardened, an ability to harden reentrant surfaces, very high speed of hardening and no separate quenching step. The disadvantage is that the hardening is shallower than in induction and flame hardening 

Carburizing 

      Carburizing is the most widely used method of surface hardening. Here, the surface layers of low carbon steel are enriched with carbon up to 0.8-1.0%. The source of carbon may be a solid medium, a liquid or a gas. In all cases, the carbon enters the steel at the surface and diffuses into the steel as a function of time at an elevated temperature. Carburizing is done at 920-950o C. This fully austenitic state is essential. If carburizing is done in the ferritic region, the carbon, with very limited solubility in ferrite, tends to form massive cementite particles near the surface, making the subsequent heat treatment difficult. For this reason, carburizing is always done in the austenitic state, even though longer times are required due to the diffusion rate of carbon in austenite being less that in ferrite at such temperatures.

 Cyaniding 

     Cyaniding is done in a liquid bath of NaCN, with the concentration varying between 30 and 97%. The temperature used for cyaniding is lower than that for carburizing and is in the range of 800-870o C. The time of cyaniding is 1-3 hr to produce a case depth of 0.25 mm or less 

 Nitriding 

       Nitriding is carried out in the ferritic region. No phase change occurs after nitriding. The part to be nitrided should posses the required core properties prior to nitriding. During nitriding, pure ammonia decomposes to yield nitrogen which enters the steel. The solubility of nitrogen in ferrite is small. Most of the nitrogen, that enters the steel, forms hard nitrides (e.g., Fe3N). The temperature of nitriding is 500-590o C. The time for a case depth of 0.02 mm is about 2 hr. In addition to providing outstanding wear resistance, the nitride layer increases the resistance of carbon steel to corrosion in moist atmospheres.