What Is Free-Cutting Steel?
Free-cutting steel generally offer low machining costs as a result of fast chipping operations. Thanks to the ease of metal working provided by this type of material, free-cutting steel is defied as the most suitable type of steel for machining operations such as longitudinal and face turning operations on multi-shaft or revolver lathe machines, thread cutting, drilling and reaming.
Classification of Free-Cutting Steels
Based on the thermal processing applied, free-cutting steel as classified in two main groups as follows:
a- Without thermal processing (soft free-cutting steels)
b- With thermal processing
b1) Tempered free-cutting steels
b2) Case-hardening free-cutting steels
Processes Applied on Free-Cutting Steels
Since precise dimensional tolerances are required for surfaces on which machining is not needed, free-cutting steel are generally used in call-drawn form. Steels defined as semi free-cutting steels with sulphur content of 0.120% and low carbon content provide good machinability and can be used for manufacturing hot and cold forged and punched materials (For example, nuts). Type 9SMn28, which has high sulphur content, can be successfully used in hot forging processes. By the way, various pipes have begun to be manufactured by pressing and after that extrusion of semi free-cutting steels. This kind of processing allows machining cost to be lowered and a great deal of material saving.
Fields of Use for Free-Cutting Steels
Since free-cutting steels provide long service life for cutting tools in high-speed lathe and machining equipments, ease of processing and a good level of chip hardness making possible to work conveniently. As a result of very low cutting force requirement, they offer high surface quality and low dimensional tolerances ("good machinability" in other words) and thus preferred by sectors manufacturing different types of joints, machineries and equipments for the automotive industry, and also by the construction industry and sectors manufacturing precise mechanical components (parts for optical and measurement devices).
What is Tempered Steel?
Tempered steels are alloyed or non-alloyed steels used in machinery manufacturing. Their composition is suitable for being hardened especially in terms of carbon content, and they provide high level of toughness at a certain value of tensile strength after the process of tempering.
Tempering process in fact is defined as the combination of hardening and tempering processes applied for providing high toughness for steel materials. Thanks to their superior mechanical properties, tempered steels has a wide area of uses including manufacturing of various machinery and engine components, forged parts, various nuts, bolts and stud bolts, crankshafts, axes, command drive parts, cylinder arms, various shafts and gears. For this reason, tempered steel has the highest figures of supply and demand after construction steels ad non-alloyed steels.
Selection of the right tempered steel for intended use and application of the right tempering process requires high level of care and experience. Obtaining good results (reaching desired toughness or hardness values) from the tempering process is closely related to the internal structure pureness of the steel used. Internal pureness means the removal of melted gases (hydrogen, oxygen and azoth) and sulphur oxide inclusions from the steel structure.
What is Case-Hardened Steel?
Case-hardened steels are alloyed or non-alloyed steels with low carbon content, which are used for manufacturing parts requiring hard and abrasion-resistant surfaces and softer, more though cores for resistance against variable and pulsed forces. These properties are brought in steel parts by impregnating steel surfaces with carbon. Case-hardened steels are used for manufacturing gears, shafts, cylinder pins, chain links, chain wheels and gears, discs, bearing strips, ball bearings, rollers, some measuring and control devices, parts subjected to medium or high level forces, parts shaped through cold heading or extrusion, cutting tools, etc. By using case-hardened steels instead of high carbon content steels with the same surface hardness offer the following advantages:
- Since the case-hardening process is applied after the part completely or partly takes its final shape, the part can be easily machined.
- If there are sections which are not intended to be hardened, these sections can be covered with a special paste of electrolytic cupper. In this case, these sections are affected by the case-hardening process and they can be easily machined afterwards. Since the core will remain soft after the case-hardening process, distortions are observed very rarely during hardening.
- Inner parts of case-hardened steels can be easily machined.
- Case-hardened steels are generally cheaper than high carbon content steels providing the same surface hardness.