5 Forging Basics You Should Know

Forging is a processing method that uses forging machinery to exert pressure on metal blank to produce plastic deformation to obtain forgings with certain mechanical properties, certain shape and size.

It is one of the components of forging (forging and stamping).

Forging can eliminate the defects such as as as cast porosity in the smelting process and optimize the microstructure.

At the same time, because the complete metal streamline is preserved, the mechanical properties of forgings are generally better than those of castings of the same material.

For important parts with high load and severe working conditions in relevant machinery, forgings are mostly used, except for rolled plates, profiles or weldments with simple shape.

1. Deformation temperature

The initial recrystallization temperature of steel is about 727 ℃, but 800 ℃ is generally used as the dividing line, and hot forging is higher than 800 ℃;

It is called warm forging or semi hot forging at 300 ~ 800 ℃, and it is called cold forging at room temperature. Forgings used in most industries are hot forging.

Warm forging and cold forging are mainly used for forging automobile, general machinery and other parts.

Warm forging and cold forging can effectively save materials.

Forging Basics

2. Types of Forging

As mentioned above, according to the forging temperature, it can be divided into hot forging, warm forging and cold forging.

According to the forming mechanism, forging can be divided into free forging, die forging, ring grinding and special forging.

1) Free forging

It refers to the processing method of forging by using simple universal tools or directly applying an external force to the blank between the upper and lower anvil of forging equipment to deform the blank and obtain the required geometry and internal quality.

Forgings produced by the free forging method are called free forgings. Free forging mainly produces forgings with a small batch.

Forging hammer, hydraulic press and other forging equipment are used to form and process the blank to obtain qualified forgings.

The basic processes of free forging include upsetting, drawing, punching, cutting, bending, torsion, dislocation and forging. Free forging adopts hot forging.

2) Die forging

Die forging is divided into open die forging and closed die forging.

The metal blank is pressed and deformed in the forging die chamber with a certain shape to obtain forgings.

Die forging is generally used to produce parts with small weight and large batch.

Die forging can be divided into hot die forging, warm forging and cold forging.

Warm forging and cold forging are the future development direction of die forging, and also represent the level of forging technology.

According to materials, die forging can also be divided into ferrous metal die forging, nonferrous metal die forging and powder product forming.

As the name suggests, the materials are ferrous metals such as carbon steel, nonferrous metals such as copper and aluminum and powder metallurgy materials.

Extrusion should belong to die forging, which can be divided into heavy metal extrusion and light metal extrusion.

Closed die forging and closed upsetting belong to two advanced processes of die forging.

Because there is no flash, the utilization rate of materials is high.

It is possible to finish complex forgings with one or several processes.

Because there is no flash, the stress area of the forging is reduced and the required load is reduced.

However, it should be noted that the blank cannot be completely limited.

Therefore, it is necessary to strictly control the volume of the blank, control the relative position of the forging die and measure the forging, so as to reduce the wear of the forging die.

3) Grinding ring

Ring grinding refers to the production of ring parts with different diameters by a special equipment ring grinding machine.

It is also used to produce wheel parts such as automobile hub and train wheel.

4) Special forging

Special forging includes roll forging, cross wedge rolling, radial forging, liquid die forging and other forging methods, which are more suitable for the production of some special shape parts.

For example, roll forging can be used as an effective pre forming process to greatly reduce the subsequent forming pressure;

Cross wedge rolling can produce steel ball, transmission shaft and other parts;

Radial forging can produce large barrel, step shaft and other forgings.

Forging Basics

5) Forging die

According to the movement mode of forging die, forging can be divided into swing rolling, swing rotary forging, roll forging, cross wedge rolling, ring rolling and cross rolling.

Rotary forging, rotary forging and ring rolling can also be processed by precision forging.

In order to improve the utilization of materials, roll forging and cross rolling can be used as the previous process of slender materials.

Like free forging, rotary forging is also locally formed.

Its advantage is that compared with the forging size, it can also be formed when the forging force is small.

In this forging method, including free forging, the material expands from near the die surface to the free surface during processing.

Therefore, it is difficult to ensure the accuracy.

Therefore, by controlling the movement direction of the forging die and the rotary forging process by computer, the products with complex shape and high accuracy can be obtained with low forging force, such as the production of forgings such as steam turbine blades with many varieties and large sizes.

The die movement of forging equipment is inconsistent with the degree of freedom, which can be divided into the following four forms:

Form of limiting forging force:

An oil press whose oil pressure directly drives the sliding block.

Quasi stroke limit mode:

An oil press that drives a crank connecting rod mechanism by oil pressure.

Stroke limit mode:

Mechanical press with slider driven by crank, connecting rod and wedge mechanism.

Energy limiting mode:

Screw and friction press using screw mechanism.

In order to obtain high accuracy, attention should be paid to prevent overload at the bottom dead center and control the speed and die position, because these will affect the forging tolerance, shape accuracy and die life.

In addition, in order to maintain the accuracy, attention should also be paid to adjusting the clearance of the sliding block guide rail, ensuring the stiffness, adjusting the bottom dead center and using the auxiliary transmission device.

There are also ways of vertical and horizontal movement of the slider (for forging slender parts, lubricating cooling and forging parts for high-speed production).

The movement in other directions can be increased by using the compensation device.

The above methods are different, the required forging force, process, material utilization, output, dimensional tolerance and lubrication and cooling methods are different, and these factors also affect the automation level.

3. Forging materials

The forging materials are mainly carbon steel and alloy steel with various components, followed by aluminum, magnesium, copper, titanium and their alloys.

The original state of the material is bar, ingot, metal powder and liquid metal.

The ratio of the cross-sectional area of metal before deformation to the cross-sectional area after deformation is called the forging ratio.

Correct selection of forging ratio, reasonable heating temperature and holding time, reasonable initial and final forging temperature, reasonable deformation and deformation speed are closely related to improving product quality and reducing cost.

Generally, small and medium-sized forgings use round or square bars as blanks.

The grain structure and mechanical properties of the bar are uniform and good, the shape and size are accurate, and the surface quality is good, which is convenient for mass production.

As long as the heating temperature and deformation conditions are reasonably controlled, forgings with excellent performance can be forged without large forging deformation.

Ingots are only used for large forgings.

The ingot is a cast structure with large columnar crystal and loose center.

Therefore, through large plastic deformation, the columnar crystals must be broken into fine grains and compacted loosely in order to obtain excellent metal microstructure and mechanical properties.

The powder metallurgy preform formed by pressing and firing can be made into powder forging by non flash die forging in hot state.

Forging powder is close to the density of general die forgings, has good mechanical properties and high precision, and can reduce subsequent cutting.

The internal structure of powder forging is uniform without segregation. It can be used to manufacture small gears and other workpieces.

However, the price of powder is much higher than that of the general bar, and its application in production is limited.

By applying static pressure to the liquid metal poured in the die bore to make it solidify, crystallize, flow, plastic deformation and shape under the action of pressure, the die forgings with the required shape and properties can be obtained.

Liquid metal die forging is a forming method between die casting and die forging. It is especially suitable for complex thin-walled parts that are difficult to form by general die forging.

In addition to common materials and aluminum, magnesium, copper, titanium and their alloys, wrought alloys of iron-based superalloys, nickel based superalloys and cobalt based superalloys are also completed by forging or rolling.

However, these alloys are relatively difficult to forge because their plastic zone is relatively narrow.

There are strict requirements for heating temperature, open forging temperature and final forging temperature of different materials.

4. Process flow

Different forging methods have different processes, among which the process flow of hot die forging is the longest.

The general sequence is: forging blank blanking → forging blank heating → roll forging blank preparation → die forging forming → trimming → punching → correction → intermediate inspection, inspection of forging size and surface defects → forging heat treatment to eliminate forging stress and improve metal cutting performance → cleaning, to remove the surface oxide scale → correction → inspection.

Generally, forgings shall undergo appearance and hardness inspection, and important forgings shall also undergo chemical composition analysis, mechanical properties, residual stress and other inspection and NDT.

Forging Basics

5. Characteristics of forgings

Compared with castings, the microstructure and mechanical properties of metals can be improved after forging.

After hot working deformation by forging method, due to the deformation and recrystallization of metal, the original coarse dendrite and columnar grain become equiaxed recrystallization structure with fine grain and uniform size, which makes the original segregation, porosity, porosity, slag inclusion and other compaction and welding in the ingot more compact, and improves the plasticity and mechanical properties of metal.

The mechanical properties of castings are lower than those of forgings of the same material.

In addition, forging processing can ensure the continuity of metal fiber structure, keep the fiber structure of forgings consistent with the shape of forgings, complete metal flow line, and ensure that parts have good mechanical properties and long service life.

Forgings produced by precision die forging, cold extrusion, warm extrusion and other processes are unmatched by castings.

Forging is an object in which the metal is pressed to shape the required shape or appropriate compressive force through plastic deformation.

This force is typically achieved by using a hammer or pressure.

The forging process builds a refined particle structure and improves the physical properties of the metal.

In the practical use of parts, a correct design can make the particle flow in the direction of main pressure.

Casting is a metal molded object obtained by various casting methods, that is, the molten liquid metal is injected into the pre prepared mold by pouring, injection, suction or other casting methods, and then cooled, sand falling, cleaning and post-treatment to obtain an object with a certain shape, size and performance.

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