Machinability, Castability, Formability, Hardenability and Weldability of Steel

Steels are defined primarily by their chemical composition, namely, that they are alloys composed of iron and other alloying elements [1]. There are many classes of steel, such as alloy steel, carbon steel, and stainless steel. The abilities of steel refer to how easily it can be handled for practical application. This is distinct from but is determined by the properties of steel, which include mechanical properties like tensile strength and hardness, thermal properties like the coefficient of thermal expansion, and others.

Here, you will learn about:

  •   The machinability of steel
  •   The castability of steel
  •   The formability of steel
  •   The hardenability of steel and how it is measured
  •   The weldability of various types of steel

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Figure 1. Steel pipes in different forms

Machinability of Steel

Machinability is defined as the ease with which a metal can be machined. Simply stated, it is the ease with which steel chips can be removed in various machining operations while retaining a satisfactory finish. Generally, the machinability of steel decreases with increasing mechanical performance.

Of the factors affecting the machinability of steel is its physical properties, such as modulus of elasticity, thermal conductivity, and hardness. The condition of steel also affects its machinability. Microstructure, grain size, heat treatment, fabrication, chemical composition, yield strength, and tensile strength determine the condition of steel.

Quantifying machinability is difficult, as there are many factors that influence it. Nevertheless, some of the criteria to be considered when evaluating the machinability of steel are presented in the table below.

Table 1. Criteria for evaluating the machinability of steel

Criteria for evaluating machinability of steel

Description

Tool life

Tool life describes how long a tool lasts and is a useful parameter for evaluating the machinability of steel. However, it is also dependent on other factors such as cutting speed, cutting tool material, cutting tool geometry, cut geometry and machine condition. A more easily machinable steel is one which permits longer tool life for a given cutting speed.

Cutting force

Steels that require higher cutting forces for machining under specified conditions are less machinable.

Surface finish

The quality of the cut edge can also be used to determine the machinability of a metal. Steels with a high strain hardening ability tend to form built-up edges during cutting, which results in poor surface finish. Cold worked steels don't tend to form built-up edges, and so are considered more machinable.

 

 

The machinability of different types of steel may be compared with standard steel by using the machinability index. It is defined as the ratio of cutting speed of the steel type being investigated for 20 minutes to the cutting speed of standard steel for 20 minutes. The SAE 1212 carbon steel is used as the standard for computing machinability index.

Table 2: The machinability index for different types of steel [2].

Steel type

Machinability index (%)

Carbon steels

42 - 170

Alloy steels

49 - 77

Stainless steel and superalloys

19 -110

Tool steels

27 - 42

Castability of Steel

The castability of steel refers to the ease of forming qualified workpieces by casting. It is affected by the properties of fluidity, shrinkage and segregation.

  • The fluidity of steel is defined as the ability of molten steel to fill mould cavities.
  • Shrinkage refers to the extent of volume reduction when the molten steel solidifies. A low rate of shrinkage is favourable for the castability of steel.
  • Segregation refers to the inhomogeneous distribution of the chemical composition of a steel object. This occurs because of the way in which steel cools while being cast: the first areas to cool are those in contact with the walls of the mould. Steel with good castability is considered to have low or negligible segregation. It can be overcome by slow cooling or subsequent heat treatment.

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Figure 2. Casting process as molten metal is poured into a mould.

Formability of Steel

The formability of steel is the ability of a steel workpiece to undergo plastic deformation without being damaged. In simple terms, it is the ability of a metal to be formed into the desired shape without necking or cracking. 

The formability of a steel type is highly dependent on its ductility, and as such, it may be evaluated by measuring the fracture strain during a tensile strength test. Steel grades which exhibit large elongation during this test have good formability. For example, A537 CL1 steel has an elongation of 22 % at 20 ⁰C. It is applicable in boilers and pressure vessel construction.

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Figure 3. Sheets of steel are rolled thanks to its formability.

Hardenability

The ability of steel to form martensite on quenching is referred to as the hardenability. It should not be confused with the hardness of steel. The hardness of steel is its ability to resist permanent deformation, while hardenability of steel is its ability to be hardened to a particular depth under some specified conditions.

The Jominy quench test is often performed to determine the hardenability of steel [3].  Here, a steel bar is machined to a specific dimension before it is heated to its austenitising temperature. This is followed by spraying a volume of water onto the end face of the bar, which in turn cools the specimen from that end.

The cooling rate varies between both ends of the steel bar. It is rapid at the quenched end and slower at the opposite end. After the specimen has been quenched, parallel flats 180⁰ apart are ground to a depth of 0.015 in (0.38 mm) along the full length of the cylindrical bar. Next, the steel specimen is marked at 1/16th intervals. A Rockwell C hardness test is performed every 1/16 in, and a curve is plotted. A typical plot of these hardness values and their positions on the steel bar is shown in Figure 4 [3]. It provides a clear indication of the depth of hardening. In addition, approximate cooling rates at designated positions can be identified from the figure.

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Figure 4. Hardness plot and cooling rate as a function of distance from the quenched end [3].

Weldability of Steel

The weldability of steel is difficult to define but is often taken to mean the ability of steel to be welded using normal processes without cold cracking occurring. The weldability of steel is inversely proportional to its hardenability. Since carbon content plays a significant role in the hardenability of steel, it also affects its weldability. Hence, as the carbon content increases, the weldability decreases. Other alloying elements such as manganese, nickel and silicon also have an effect on steel's weldability. However, their effect is not as significant as the presence of carbon content.

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Figure 5. Welding of two steel tubes.

 

Other factors affecting the weldability of steel are thermal conductivity, melting point, electrical resistivity, and the coefficient of thermal expansion.

  1. Thermal Conductivity: Steel types having low thermal conductivity are usually very easy to weld.
  2. Melting point: The lower the melting point of a particular steel type, the easier it is to weld.
  3. Electrical resistivity: Since heat energy is essential for the welding process, steel types with high electrical resistance are usually more difficult to weld by electrical means. 
  4. Coefficient of thermal expansion: When welding two metals, it is very important to consider their coefficients of thermal expansion. If the difference between the two coefficients is significant, tensile and compressive strain upon cooling can cause cracking.

Steel comes in four groups based on chemical composition: carbon steel, alloy steel, stainless steel and tool steel. These classes of steel are presented in the table below.

Table 3. Weldability and application of different classes of steel.

Type

Weldability

Application

Low carbon steel

Contains less than 0.3 wt.% carbon and 0.4 wt.% manganese. Also exhibits good weldability as long as impurities are kept low. Any welding process is suitable for low carbon steel.

It is suitable for decorative products such as lamp posts. An example is the BS 970-1 Grade 07M20 normalized steel. It has a tensile strength of 430 MPa and a yield strength of 215 MPa at 20 ⁰C.

Medium carbon steel

Contains 0.3 - 0.6 wt.% carbon and 0.6 - 1.6 wt.% manganese. The higher carbon content makes it prone to cracking. Hence, these are more difficult to weld. The low-hydrogen welding process is suitable for medium carbon steel.

It is suitable for automotive components. An example is the AISI 1541 cold drawn steel.

High carbon steel

Contains 0.6 - 1.0 wt.% carbon and 0.30 - 0.90 wt.% manganese. It also has poor weldability and cracks easily. Low hydrogen fillers must be used when welding these steels.

It is used in making knives, axles, and punches. An example is the AISI 1080 hot rolled steel.

 

Alloy Steel

Alloy steels often have higher hardness compared to other categories of steel. Hence, they also possess poor weldability and are prone to cracking. Low-hydrogen welding process must be used for alloy steels. During the welding process, attention must be paid to preheating, cooling rate and post-weld heat treatment since alloy steels are also prone to cracking.

Alloy steels have different mechanical properties based on chemical composition. They are used in the manufacture of pipelines, electric motors and power generators.

An example is the AISI 8620 normalized steel. It has a yield strength of 360 MPa and a tensile strength of 640 MPa at 20 ⁰C.

Stainless Steel

Stainless steels are a group of alloy steels. They contain at least 10.5 wt.% chromium and other elements which improve their heat resistance and improve mechanical properties [4].

Austenitic stainless steel has good weldability and requires no pre or post-weld heat treatment. The ferritic type undergoes rapid grain growth at high temperature, which makes them brittle. Hence they have poor weldability. 

Austenitic steels are used in the manufacture of pipes, kitchen utensils and other food processing equipment.

Ferritic steels are also used in automotive applications and industrial equipment.

The SUS 321 stainless steel supplied by TJC Iron & Steel Co., Ltd is suitable for pressure vessels.

Tool steel

Tool steel contains up to 2.5 wt.% carbon. It has poor weldability.

Used for cutting and drilling equipment. The DIN 17350 Grade C105W1 soft annealed steel is suitable for making tapper, dies, mandrels, and hammers.

The future of steel processing

Discovering new ways of working with steel is a huge area of research in materials science and engineering. As an example, there has recently been a boom in interest in additive manufacturing of steel components [5]. This process can be used to efficiently manufacture steel components with complex geometries at a reduced cost.

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Figure 6. Additive manufacturing of steel parts. (TRUMPF)

Sources

[1] G. Krauss, Steels. Materials Park, Ohio: ASM International, 2010, p. 2.

[2] Machinability Index Table Chart for Steel, Aluminum, Magnesium, Cast Iron, Carbon Steel, Alloy Steel and Stainless Steel - Engineers Edge", Engineersedge.com, 2020. [Online]. Available: https://www.engineersedge.com/materials/machinability-steel-aluminum.htm.

[3] J. Dossett, Steel heat treating fundamentals and processes. Materials Park, OH: ASM International, 2014, p. 29.

[4] J. Lippold and D. Kotecki, Welding metallurgy and weldability of stainless steels. Norwood Mass.: Books24x7.com, 2005, p. 5.

[5] A. Zadi-Maad, R. Rohib and A. Irawan, "Additive manufacturing for steels: a review", IOP Conf. Series: Materials Science and Engineering, 2017. Available: 10.1088/1757-899X/285/1/012028.

Definitions

 

Machinability: is the ease with which a metal can be machined.

 

Castability: is the ease of forming qualified workpieces by casting.

 

Formability: is the ability of a steel workpiece to undergo plastic deformation without being damaged.

 

Hardenability: is the ability of steel to form martensite on quenching is referred to as the hardenability.

 

Weldability: is the ability of steel to be welded using normal processes without cold cracking occurring.

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