Heat treatment of Forgings – Introduction

Heat at treatment is a process involving heating of a metal/alloy at a specific rate, holding it at a temperature for a period of time and cooling it at a specific rate. Heat treatment is performed to obtain a desired microstructure and achieve certain physical, mechanical, magnetic or electrical properties.
Components produced by forging are heat treated to obtain desired microstructure and specified properties.

Objectives of heat treatment:
The major objectives are
-To increase strength, hardness and wear resistance
-To increase ductility and toughness.
-To obtain fine grain size by recrystallization annealing, full annealing or normalizing.
-To remove residual internal stresses formed due to cold working by stress relief annealing
-To improve machinability of forged components
-To improve cutting properties of tool steels
-To improve surface hardening, corrosion resistance, high temperature resistance by precipitation hardening
-To improve electrical properties
-To improve magnetic properties by phase transformation, of the forged components
There are various types of heat treatment processes which are performed separately or as a combination to get the desired properties and microstructure after forging.
-Annealing
-Normalizing
-Stress Relieving
-Precipitation hardening
-Quenching
-Tempering
-Case Hardening
Annealing – Heating of the alloy above critical temperature and allowing to cool in the furnace itself after switching off the furnace is called annealing. This increases the ductility and toughness but reduces the hardness.
Normalizing – Heating of alloy above critical temperature, soaking it at that temperature and cooling it in air is called normalizing. This increases the strength and hardness. This process is performed on forging components that are to be machined as normalizingimproves the machinability of forged components.
Stress Relieving – Forging, rolling, cold working, non-uniform cooling cause internal stresses in a metal. To remove or reduce these internal stresses created in a metal, stress relieving is done by heating a metal below the lower critical temperature, followed by uniform cooling.
Quenching – Heating of the alloy above critical temperature, holding it at that temperature for a specified period of time and cooling it in water, oil, salt bath or organic solvent, such that it cools rapidly is called quenching or hardening. This increases the hardness but deteriorates the toughness and increases the brittleness, making it unsuitable for machining. So, forged components are tempered after hardening.

Tempering – Heating of alloy below the lower critical temperature (e.g., for steel, around 300ᵒC-450ᵒC) soaking it for a specified period of time and cooling it in air is called tempering. This reduces the brittleness and increases the ductility and toughness. Thus, making the forged components suitable for machinability and increasing their shock absorbance.

Precipitation Hardening – Strength and hardness of some metals can be improved by formation of second phase particles dispersed in the parent matrix. These new phase particles are called precipitates. These precipitates must be coherent to the parent matrix. These precipitates act as barriers to the dislocation motion, thus increasing the stresses required to overcome these barriers eventually hardening and strengthening the alloy.
Case Hardening – Some components require higher hardness at surfaces than at cores. For such components surface hardening is done which hardens only the surface/ case. This is done to improve the wear and tear resistance of the component. Usually automobile gears produced by forging are case hardened.

The above are the common heat treatment processes performed on forgings. The temperature and type of process may vary according to the component, composition and properties required.

Applications of Forging and their Importance – Forging Process

As mentioned in Application of Forgings and their importance, here is the detailed article on Application of forgings

AUTOMOBILES:

Strength, reliability and economy are the desired characteristics for application in automobile industry. Forged components possess these characteristics, thus making forged parts suitable for application in automobile industry. Forged components possess good shock absorbing capacity and therefore are found at points of shock and stress such as stub axle beams, wheel spindles, shafts, kings pin, torsion bars, ball studs, pitman arms, idler arms, and steering arms.

Forged parts also find application in the power train, where connecting rods, drive shafts, transmission shafts, differential gears, clutch hubs and yokes are often forged.

VALVES AND FITTINGS:
Forgings are often associated with the high pressure applications in the valve and fitting industry because of their superior mechanical properties and freedom from porosity. Corrosion and heat-resistant materials are generally used for valve bodies and stems, ss flanges, tees, saddles, elbow reducers, and other fittings.

HAND TOOLS & HARDWARE
Hand tools and hardware have been traditionally forged as the mark of quality. Wrenches, pliers, hammers, garden tools, sledges, wire-rope clips, sockets, nut and bolts, hooks, turnbuckles are common examples of forged hand tools and hardware. Forged components find application in surgical and dental instruments too.

MACHINERY & EQUIPMENT
Forgings find application in off-highway and heavy construction equipment, mining equipment, and material handling applications owing to their high strength, toughness and mach inability. Equipment and machinery used in steel, paper, textile, power generation and transmission, chemical and refinery industries also use forged components.

DEFENCE AND ARMED FORCES: 
Forged components are found in almost every implement of defence, from rifle triggers to nuclear submarine drive shaft, all comprise forged parts. Missiles, heavy tanks, armored carriers, shells and other heavy weapons are common defence-related applications of forged components.

AEROSPACE
For aerospace application, high strength-to-weight ratio and structural reliability are the most significant properties required. Performance, range, and payload capabilities of aircraft highly depend on these properties. Commercial jets, helicopters, piston-engine planes, military aircraft and spacecraft widely use forgings. Forgings component are used as and in bulkheads, wing roots and spars, engine mounts, hinges,  brackets, shafts, beams, landing gear cylinders and struts, wheels, brake carriers and arresting hooks. In jet turbine engines, super alloys (iron-base nickel-base and cobalt-base) are forged into components such as buckets, discs, blades, couplings, rings, manifolds, chambers and shafts.