Forging vs. Rolling: The Differences Explained

Forging and rolling are two of the most common metal processing techniques used in the manufacturing industry. Both methods involve applying pressure to metal to create a desired shape, but each has its own unique advantages and disadvantages.

In this article, we explore the differences between forging and rolling, including the types of each technique, their respective advantages and disadvantages, and the impact on the mechanical properties of the final product.

We also delve into the various types of forging and rolling, such as longitudinal, cross, and skew rolling, as well as free forging, die forging, and flashless die forging.

Whether you’re a seasoned industry professional or simply curious about the manufacturing process, this article provides a comprehensive overview of forging and rolling, including the factors that determine which method is best for a particular application.

So, whether you’re looking to improve the quality of your steel products, optimize your manufacturing process, or simply expand your knowledge of the industry, read on to discover the differences between forging and rolling and how they can impact your bottom line.

Rolling

What is rolling?

A pressure processing method in which a metal blank is passed through the gap between a pair of rotating rolls with various shapes, causing the cross-section of the material to decrease and its length to increase as a result of the compression from the rolls. This method is the most commonly used production technique for steel and is mainly used to produce profiles, plates, and tubes.

Types of rolling

According to the movement of rolled parts, rolling can be divided into longitudinal rolling, cross- rolling and skew rolling.

Longitudinal rolling

The longitudinal rolling process is a process in which metal passes between two rolls that rotate in opposite directions and produces plastic deformation between them.

Cross-rolling

The movement direction of the rolled piece after deformation is consistent with the roll axis direction.

Skew rolling

The rolling piece moves in a spiral, the rolling piece and the roll axis are not with a special angle.

Advantages

The rolling process can improve the quality of steel by destroying the casting structure of the steel ingot, refining the grain of the steel, and eliminating microstructural defects. This leads to a denser steel structure and improved mechanical properties, particularly in the rolling direction.

Additionally, high temperature and pressure during rolling can weld together any bubbles, cracks, or looseness that may have formed during casting.

Disadvantages

1. Delamination after rolling: The non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are compressed into thin sheets, resulting in the delamination phenomenon. This greatly decreases the tensile properties of the steel in the thickness direction and may lead to interlayer tearing during weld shrinkage. The strain induced by the weld shrinkage can often be several times the yield point strain, much higher than the strain caused by the load.
2. Residual Stresses due to Uneven Cooling: Residual stress is internal stress that is in self-equilibrium without external force. Hot-rolled steel products of various cross-sections have this kind of residual stress, which tends to increase with the size of the beam’s cross-section. Although the residual stress is self-equilibrium, it can still impact the performance of the steel component when subjected to external forces, affecting its deformation, stability, and fatigue resistance.
3. Inaccurate Dimensions: Hot-rolled steel products are challenging to control in terms of thickness and edge width. Thermal expansion and contraction during the cooling process can result in a difference between the initial and final length and thickness. The bigger the difference, the thicker the steel and the more obvious the discrepancy. Therefore, it’s not possible to be too precise about the width, thickness, length, angles, and edge lines of large steel components.

Forging & Pressing

Forging is a processing method that utilizes forging and pressing machinery to apply pressure to metal billets, resulting in plastic deformation and the creation of forgings with specific mechanical properties and shapes.

This process eliminates casting defects and optimizes the microstructure of the metal during the smelting process. The preserved integrity of the metal flow lines leads to better mechanical properties in forgings compared to castings made of the same material.

Forgings are commonly used for important parts with high load and harsh operating conditions, as well as for simple shapes that can also be created from rolled plate, profile, or welded parts.

Types of forging

Forging can be divided into three categories: free forging, die forging, and flashless die forging.

1. Free forging: This type of forging uses either impact or pressure to deform the metal between the upper and lower iron, also known as anvil, to obtain the desired shape. It can be further divided into manual forging and mechanical forging.
2. Die forging: This type of forging is divided into open-die forging and flashless forging. The metal blank is compressed and deformed in a forging die with a specific shape to produce forgings. It includes cold heading, roll forging, radial forging, and extrusion, among others.
3. Flashless die forging and closed upsetting forging: In this type of forging, there is no flash, which results in a high material utilization rate. Complex forgings can be completed in one or several processes, and the force-bearing area of the forging is reduced, thus reducing the required load. However, it is important to note that the blanks cannot be completely restricted, and so the volume of the blanks must be precisely controlled, the position of the forging dies must be monitored, and efforts should be made to reduce die wear.

Features

Compared to castings, forging can improve the structure and mechanical properties of metal. During the forging process, hot working deforms and recrystallizes the casting structure, causing coarse dendrites and columnar grains to transform into a finer and more uniform equiaxed recrystallized structure.

The forging process also compacts and welds impurities such as segregation, porosity, and slag inclusions, leading to a tighter structure and improved plasticity and mechanical properties.

The mechanical properties of castings are generally lower than those of the same material in forgings. Furthermore, the forging process ensures the continuity of the metal’s fiber tissue, preserving the consistency of the forgings’ shape and metal flow integrity.

Precision die forging, cold extrusion, and temperature extrusion processes can produce forgings with excellent mechanical properties and long service life, which is unmatched by castings.

Forging vs Rolling

(1) The mechanical properties of forgings in the axial and radial directions are more consistent compared to rolled products.

This means that forgings have a much higher degree of isotropy, resulting in a longer lifespan compared to rolled products.

The figure below illustrates the metallographic diagram of eutectic carbides in different directions of a Cr12MoV rolled sheet.

(2) In terms of the degree of transformation, the deformation degree of forging is much greater than that of rolling, meaning that forging is more effective than rolling in breaking eutectic carbide.

(3) In terms of processing cost, forging is much more expensive than rolling.

For key parts, workpieces subjected to high loads or impacts, and workpieces with complex shapes or strict requirements, forging must be used.

(4) Forged parts have complete metal flow lines.

Mechanical work after rolling destroys the integrity of metal flow lines, significantly shortening the life of the workpiece.

The picture below shows the metal flow lines of casting, machining, and forging workpieces.