How to distinguish between cold working and hot working? Why do forgings have better mechanical properties than castings?

In the field of metal working,cold working and hot working are two distinct process methods.They not only have significant differences in the production process,material deformation characteristics,finished product quality,etc.,but also directly affect the final performance and application field of the product.This article will delve into the differences between cold and hot working and explain why forgings are generally superior to castings in terms of mechanical properties.

一．Definition and characteristics of cold processing and hot processing

Cold working

Cold working usually refers to the process of plastic deformation of metal at a lower recrystallization temperature.This process includes cold rolling,cold drawing,cold forging,stamping,cold extrusion and other methods.The main feature of cold working is that the deformation resistance is large,and the hardness and strength of the workpiece can be improved by work hardening while the metal is formed.

Cold machining is usually suitable for metal parts with small section size and high processing size and surface roughness requirements.Because cold working can cause high residual stress inside the material,while weakening ductility and electrical conductivity,it is not suitable for use in applications requiring high ductility or electrical conductivity.In addition,the effect of cold processing will decrease or even disappear in high temperature environments,so parts in high temperature environments are generally not strengthened by cold processing.

In mechanical manufacturing technology,cold working also refers to metal cutting,such as turning,drilling,milling,planing,grinding and so on.These machining methods use cutting tools to remove excess metal layers from metal materials or workpieces to obtain a machined surface with a certain shape,dimensional accuracy and surface roughness.The productivity of machining is generally higher,but in special occasions,its productivity may be lower than that of precision casting,precision forging and powder metallurgy.

Hot working

Hot working refers to the process of plastic deformation and recrystallization of metals at higher than recrystallization temperature.This process mainly includes casting,hot rolling,forging and metal heat treatment.Sometimes,welding,thermal cutting,thermal spraying and other processes are also classified as thermal processing.

Casting is the liquid metal or molten metal alloy poured into the mold,after cooling solidification and demoulding to obtain metal parts.Casting can produce parts with complex shapes,but due to the defects such as pores and inclusions,these defects may affect the mechanical properties and durability of castings.

Hot rolling is the process of forming the metal after heating it to the plastic deformation stage.Hot rolled steel plate has low hardness,easy processing and good ductility,but the strength is relatively low and the surface quality is slightly poor.Hot rolled plates are usually medium and thick plates and are suitable for applications where strength and surface quality are not required.

Forging is the use of forging hammer,press and other equipment to forge or extrude the metal blank,so that it produces plastic deformation,so as to obtain the required shape and size of the metal products.During the forging process,the metal blank is subjected to pressure,the internal structure is densified,and the mechanical properties are significantly improved.The strength,hardness,wear resistance and tensile strength of forgings are usually better than those of castings.

Metal heat treatment includes annealing,normalizing,quenching,tempering and other processes to change the internal structure of the metal,thereby improving its mechanical properties.Annealing can reduce the hardness of metal,improve plasticity,is conducive to cutting or pressure processing;Normalizing makes the structure finer and is often used to improve the machinability of the material.Quenching can improve the hardness,strength and wear resistance of the workpiece;Tempering can eliminate the stress generated by quenching,so that the steel has both hardness and toughness.

二.Comparison of mechanical properties of forgings and castings

There are significant differences between forgings and castings in manufacturing process,shape and structure,performance and quality,which directly affect their mechanical properties.

Manufacturing process

Castings are metal parts obtained by pouring liquid metal into a mold and solidifying after cooling.The process is similar to pouring melted wax into a mold to make a wax figure,but the material is changed to metal.Casting is capable of producing parts with complex shapes,including curved,thin-walled,uneven structures,but its shape and size are mainly determined by the mold.

Forging is the use of forging hammer,press and other equipment to forge or extrude the metal blank,so that it produces plastic deformation,so as to obtain the required shape and size of the metal products.The forging process is similar to how dough is molded into various shapes in the hands of a cook.Although forgings can also produce complex shapes,their shape and size are less flexible than castings.

Shape and structure

Casting can produce a variety of complex shapes of parts,including curves,thin walls,uneven structures.These complex shapes and structures give castings unique advantages in certain application scenarios.However,castings are prone to defects in the manufacturing process,such as pores,inclusions,etc.,which may affect the mechanical properties and durability of castings.

Although forging can also produce complex shapes,its shape and size flexibility is less than that of castings.However,when the metal blank is subjected to pressure during forging,its internal structure is densified and its mechanical properties are significantly improved.The strength,hardness,wear resistance and tensile strength of forgings are usually better than those of castings.

Performance and quality

The defects such as pores and inclusions occur easily in the manufacturing process of castings,which may affect the mechanical properties and durability of castings.In addition,the structure of the casting is usually relatively large,with shrinkage holes,shrinkage and other defects,which further reduce the mechanical properties of the casting.

In the forging process,the metal blank is subjected to pressure,its internal structure is densified,and its mechanical properties are significantly improved.The strength,hardness,wear resistance and tensile strength of forgings are usually better than those of castings.At the same time,the dimensional accuracy and surface quality of the forging are relatively high.Forgings obtained by pressure processing,fine organization,uniform grain,eliminate a variety of organizational defects,so it has higher mechanical properties and performance.

Cost and efficiency

Because castings can be produced in large quantities and the manufacturing process is relatively simple,the production cost is low and the production efficiency is high.However,the disadvantages of castings in terms of mechanical properties,dimensional accuracy and surface quality limit their application in scenarios requiring high performance.

The production cost of forgings is relatively high,because the forging equipment is expensive,and the forging process requires a lot of energy and manpower.However,the high performance and high quality of forgings can often compensate for this cost disadvantage.Forgings are more suitable for occasions where mechanical properties,dimensional accuracy and surface quality are required.

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三.Conclusion

In summary,cold working and hot working have their own unique advantages and application scenarios in the field of metal working.Cold working improves the hardness and strength of the workpiece through work hardening,which is suitable for metal parts with small section size,high processing size and surface roughness requirements;Hot processing improves the internal structure of metal through plastic deformation and recrystallization,which is suitable for metal parts with complex shapes and high mechanical properties.

There are significant differences between forgings and castings in manufacturing process,shape and structure,performance and quality.Forgings obtained by pressure processing,fine structure,uniform grain,mechanical properties are significantly improved,usually better than castings.Therefore,forgings are more suitable for occasions where mechanical properties,dimensional accuracy and surface quality are required.

In the field of metal working,the correct choice of cold working or hot working methods,as well as the reasonable selection of forgings or castings,is of great significance for improving product quality,reducing production costs,and meeting application needs.With the progress of science and technology and the continuous innovation of the process,cold processing and hot processing technology will continue to be optimized and improved,and inject new vitality into the development of the metal processing field.