General Information:

The case hardened steels are the low-carbon, alloyed or un-alloyed steel used for the production of parts required to be hard and impact-resistant on the surface and softer and more dense in the core, rendering them resistant to the variable forces under impact.
Imparting those properties to the part is done by having the surface absorb carbon. The case hardened steels are used in the production of the gears, shafts, piston pins, chain links, chain gears and rollers, disks guide bearings, roller bearings, rollers, some measurement and control devices, medium-force parts and cutting tools.
The utilization of the case hardened steels provides the following advantages compared to the utilization of the carbon steels offering the same hardness value on the surface:
- As the case hardening operation is applied after the part takes its final shape partially or completely, machining the part is quite easy,
- If there might be some areas on the surface of the piece, which do not need to be hardened, these areas are coated with a special paste or electrolytic copper. As the case hardening operation will not act on those areas, they can easily be machined afterwards.
- Because the core zone will retain its softness after the case hardening operation, the distortions that could take place during hardening are very few,
- The interior of the case hardened steels can be worked easily,
- The case hardened steels are less expensive than the other carbon steels of equal surface hardness, commonly used for machine tools,

However, selecting the appropriate case hardened steel and the proper case hardening operation requires great care and experience. Obtaining good results from case hardening (i.e. obtaining the desired hardening depth and hardness) is closely associated with the cleanness of the internal structure of the steel used.
The cleaning of the internal structure is the operation of purifying the molten steel from various gases (hydrogen, oxygen and nitrogen) dissolved inside the steel and cleaning out the sulphur oxide inclusions.
The case hardened steels are covered under two groups as,
- Quality steels (un-alloyed)
- Alloyed steels

In general, the hardening depth may be between 0.2 and 1.5 mm.
The case hardened steels are also divided in to three groups as low hardness, medium hardness and high hardness. The carbon percentage in the case hardened steels vary between 0.10 and 0.20

The most prominent characteristic of the steels in this group is: the density property due to the low carbon content. Higher C percentage will weaken the density if hardened too deep.
The low carbon steels have lower structural density but are used where high density is required, whereas the high-carbon types are used where high structural density is needed.
Higher alloy steels should be used and the cross section increased.
These are called the "gear steels" due to the purpose of utilization. The important properties for the gear teeth are being highly resistant to wear and fatigue.
The fatigue life increases as the surface hardening depth is increased. One factor that affects the fatigue resistance most is the "C" content.
Those properties are imparted through the case hardening process.

Nitridable steels:

Definitions;
Nitridable steels are the perlitic-martensitic steels, which are particularly suitable for nitriding, due to the nitride instigator in their composition.
Nitriding: is the process of keeping the steels of suitable composition in an environment with a nitrogen-emitting gas such as ammonia at 490-520°C temperature.
Nitdiring is applied in order to increase the surface hardness and resistance against wear, rust and fatigue.

Induction hardened steels:

This group of steel covers all the treated steels. Their special characteristic is indicated with the letter (F), meaning reduced phosphorus, such as CF 53.
This group of steels are made dense and surface hardened by surface hardening process after treatment.
The surface defects and decarburizing reduces the surface hardness. Therefore, grinding beforehand is recommended. Furthermore, annealing at about 200°C temperature after hardening should not be forgotten.

Although the Cf 45-Cf 53-42Cr4-4lCrMo4-49CrMo4 type steels are the most prominent variety, the DIN 17200-conformant (treated steels) steels also possess the same properties.

Roller and Bearing Steels

Their production requires great care at each stage. The best quality is obtained by casting under vacuum. Following the rolling operation, they are subjected to the spherical annealing process and delivered.

The steels of this group are divided in to several sub-groups according to the nature of the heat treatment:

1- Direct hardened steels (such as 100 Cr 6)
2- Case hardened types
3- Treated types
4- Stainless types

OTHER STEEL TYPES
As the steel groups such as those indicated below belong to special areas, those are not included in this booklet:
1- Electrode steel,
2- Chain steel,
3- Valve steel,
4- High-manganese steel,
5- High-temperature resistant steel,
6- High-speed steel.

Bolt and nut steel:
These steels, which represent the widest characteristic group of the fastening components, can be examined in several groups:

Group 1: Those are low-carbon steels included in the DIN 1711 standard and used for manufacturing the bolts and nuts requiring low resistance. This group is similar to the mass steels and can be shaped by machining. Not much heat treatment is applied. Their cold rolled variety is known and used as TRANSMISSION in the market

Group 2: those steels are mostly for machined production, which are the Automat Steels included under DIN 1651

Group 3: those are bolt steels included under DIN 1654 with the most important characteristic of cold forged shaping. These are particularly suitable for the parts produced using cold processes, such as bolts. Therefore, the steels of tHis type have to be produced with special and strict supervision to ensure the internal structural and surface cleanness. The P content inside is limited and are indicated with the symbol (cq35). Those subjected to the spherical annealing process for cold forging.

Group 4: Those are heat-resistant bolt materials specified by the DIN 17240 standard. This group covers the Cr-Mo and Cr Mo V steels used for manufacturing the fastening elements employed at temperatures between 300-600°C

In general, the bolt steels are provided either in coils for the automated machinery or cold rolled for machining, and some with the scales pealed. The bolt and nut classes according to ISO 898 and their approximate equivalents are given below:

Class --------- Usable steel
4.81/6.8 --------- 1015/1020 (St 37 - St 42)
8.8 --------------- (Bo), (Mn) added C steels C25B, 2IMnB4
10.9 -------------- (Bo), Mn, Cr added C steels or 41cr4
12.9 -------------- TREATED alloy steel 41 cr4, 42crmo4

IMPORTANT NOTE:
Today the cq35 steels are abandoned and steel type (B) such as I9MnB4/21MnB4 C25B is used.