Forging quality steels
Forging quality steels
Forging quality steels are those steels which are subjected to the process of forging during its subsequent processing for the production of end use products. The process of forging consists of converting the steel material into designed shape at a higher strain rate. Forging quality steels have the property of forgeability which is the relative ability of the steel to flow under compressive loading without fracturing. Except for resulphurized and rephosphorized grades, most carbon and low alloyed steels are usually considered to have good forgeability. Difference in forging behaviour among the various grades of steel is small enough and hence selection of steel for a forging is seldom affected by the forging behaviour.
The forging process can be of the following three types.
- Hot forging – In this process the forging operation is usually done at a temperature of around 1200 deg C.
- Warm forging – This process is carried out below the recrystallization temperature of steel normally at temperatures ranging from 650 deg C to 750 deg C.
- Cold forging – This process is performed at room temperature and the steel material is not heated.
During the process of forging, since high strain rates are employed, the qualities needed in forging steels are critical and demanding. Further forging components demand specialized treatments necessary for imparting special properties based on the end application of the forgings. Also since the end use of the forged steel products is of critical nature, a close control over all the stages of steel manufacturing process is required.
Selection of steel for forging
Selection of a type of steel for a forged component is an integral part of the forging process and accepted performance of the steel after forging is dependent on this choice. A thorough understanding of the end use of the finished part will serve to define the required mechanical properties, surface finish requirements, tolerance to non metallic inclusions, and the attendant inspection methods and criteria.
The selection of steel for a forging normally requires some compromise between opposing factors. These opposing factors are strength versus toughness, stress – corrosion resistance versus weight, manufacturing cost versus load carrying capacity and the cost of steel versus the cost of forging etc. Selection of steel involves consideration of steel making practices and chemical and mechanical properties of steel besides consideration of forging and post forging processes which the end product has to undergo.
Forging steels are required to have the following metallurgical properties.
- Good surface quality
- Good cleanliness
- Good internal quality
- Good impact toughness
- Fine and uniform grain size of austenitic grains
- Good response to heat treatment
Purchaser of forging quality steel as per his needs specifies one or more special quality restrictions for the steel. These restrictions bring into effect additional testing requirements for the producer of this steel.
Steels for forging quality are produced to a wide range of chemical compositions by basic oxygen process and electric arc furnace steel making processes. With each of the melting and rolling practices, a level of testing and evaluation quality is exercised. The details of testing and quality evaluation vary from producer to producer and should be a point of enquiry when the forging steels are ordered.
Forging quality steels are produced in the following major categories
- Plain carbon steels
- Carbon manganese steels
- Molybdenum manganese steels
- Plain chrome steels
- Chrome manganese steels
- Chrome molybdenum steels
- Chrome nickel steels
- Chrome nickel molybdenum steels
Selection of steels for forgings
Carbon, micro-alloy and alloy steel forgings account for the greatest volume of forgings for a very wide range of applications. Stainless steels are widely used where resistance to heat and corrosion are required, in applications up to approximately 510 deg C. The material that is selected for a forging application must be one that can achieve the required physical and mechanical properties. Where alloys from several groups meet performance requirements, the most economical alloy, in terms of material and processing costs, should be chosen.
The following summary is helpful in making a preliminary selection.
Carbon, micro-alloy and alloy steels are low to moderate in cost. The main cost drivers are processing and machining. The alloy steel are readily hot forged and some shapes are cold forgeable in selected alloy steels.
Alloy formulation may also be governed by product dimensions. As section sizes become progressively heavier, higher alloy levels are required to achieve hardenability. Stainless steels are higher in cost than carbon, micro-alloy and alloy steels. They are hot forgeable into simple shapes and low profile structural shapes, but the high forging pressures restricts net shape forging to simpler shapes. The hot die forging process should be reviewed when more complex shapes are encountered or when the more difficult to forge alloys are specified. The 300 series stainless steels require 20 to 40% higher forging pressures than the 400 series of stainless steels, due mainly to the higher nickel content.
Uses of forging quality steels
These steels find wide applications in automobile industry, defense equipments, railways and manufacturing industries. Some application of forging quality steels is shown in Fig 1.
Fig 1 Application of forging quality steels