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The main purpose of case hardening process is to provide low carbon steel with superficial high carbon content, without affecting the inner or the core of the metal in its composition.
The procedure involves addition of high carbon percentage to the outer layer of a low carbon steel which is to be case hardened through heating while in contact with a certain material which may be very rich in carbon content. The procedure also involves a final heat treatment after the above procedure has been completed.
The process particularly becomes beneficial because it attributes the low carbon steel with outer hardness for resisting external shocks, hits, scratches tampering etc. at the same time the overall inner structure of the metal keeps it tough and flexible so that it stays unaffected from fractures and bending stresses.
The fundamental operations involved with low carbon case hardening steel procedure are as follows:
1. Cementing or Carburizing – which helps introducing additional carbon.
2. Gradual Cooling of the Cementing Temperature also called Annealing.
3. Refining – for compensating the Effects of Sustained Heating.
5. Tempering as Required.
The different methods employed for the above procedures are:
Open Hearth Process: The process is utilized with single part metals only. The job is heated until it becomes cheery in color, then its immersed in a carburizing compound like bone charcoal and charred leather, wood charcoal. The induction is done more than couple of times.
The metal is then cleaned from any adhering compound, re-heated, re-dipped and hardened through water cooling.
The finished metal’s hardness is tested by filing it with a smooth file and then finally polished with emery paper.
Box Process: This process becomes useful when more than one item is required to be case hardened.
All the parts to be hardened are neatly packed in an iron box. It is ensured that each and every metal and its external parts are covered and surrounded by one to two inches of carburizing compound.
The iron box opening is covered with a lid and the lid is sealed using special fire clay. This is done for preventing gases from escaping the iron box during the process of carburizing and also stops external gases from the furnace from entering the box.
The box is gently placed inside the furnace and heated up to the carburizing temperature of 900 degrees Celsius. This temperature is maintained for a sufficient period of time for optimum curing of the case thickness.
The above carburizing procedure when extended up to four hours produces a hardened case of approximately 0.040" thick.
The thickness is generally considered to be good for most of the ordinary work and permits good grinding limits.
Next, the metals are allowed to get cooled gradually inside the box itself so as to anneal the newly cured case.
Further refinement is carried by heating the jobs between 840 to 900 degrees Celsius and introduced into water or oil for quenching.
After this, ultimately the outer skin of the metals are heated to a temperature of 760 degrees Celsius and cooled to 700 degrees Celsius before quenching them back into water or oil.
Cyanide Process: The process involves dipping of the metal to be case hardened into a molten solution of sodium cyanide at 920 degrees Celsius. The process ensures both sufficient heating as well as a carburizing agent. The process may provide a hardened crust of around 0.008" to 0.01" and used for jobs where this figure proves sufficient. The articles here are either hardened directly or brought across normal refining and hardening heat treat methods.
The above process is considered to be quicker and efficient and normally can be completed within half an hour. Small metallic parts like nut, bolt, machine gears become ideally suited here. The chemical reactions involved during cyaniding may be written as:
2NaCN + 2NaCNO → 2NaCNO
2NaCNO + O2 → NaCO3 +CO + 2N
2CO → CO2 + C
Nitriding Process: The hardening produces by incorporating this method results very hard surface over special steel alloys. The formation takes place when the procedure is carried out in an environment high in ammonia gas for as long as 90 hours of time. The procedure involves metals to be hardened enclosed inside a box and heating them up to 500 degrees Celsius. Dering heating a stream of ammonia gas is allowed into the box for a period of 10 to 90 hours.
The thickness of the hardened case may vary for different type of steels, ranging from 0.006" to 0.030". The thickness may also get influenced by the period of time for which it has been ammonia treated and also the type of steel used.
Since the process eliminates the use of high temperatures, does not necessitates quenching and therefore the risk of distortions in the metal structure is reduced to great extents.