TEMPERING:
The hardening treatment specified to steel raises the hardness but initiates internal stresses due to different cooling rates. The internal stresses are created also due to transformation from austenite to martensite. Tempering treatment intends at decrease these stresses. The treatment comprises in heating the hardened component to in between 200oC and 600oC and holding this at such temperature for a predetermined period of time and after that cooling gradually to room temperature. Because martensite itself is meta-stable phase, structural changes induced via tempering proceed fairly quickly. Each structures resulting from tempering are termed martensite. Modifies occurring during several temperature ranges are explained below as:
100o - 220oC
Extremely little change happens in the micro-structure. Though, this heating assists remove considerable internal amount of stresses. The stress relieving treatment is known while maximum hardness is needed and brittleness is not a problem. The strain is relieved due to removal of carbon atoms from their trapped locations.
240o - 400oC
During this range martensite decomposes fast into emulsified form of pearlite termed as secondary troostite. This material is extremely fine in nature and thus offers good shock resistance. The suitable edge tools are tempered in such range but more exactly inside 270oC-300oC.
400o - 550oC
The precipitate troostite initiates for coalesce forming a coarser from of globular pearlite termed as sorbite. This may be recalled both sorbite and troostite are here preferably called tempered martensite. This treatment is required in such components as springs, axles and beams.
600o - 700oC
Heating hardened steel during such range causes spheroidisation, the structure being termed as spheroidite. Such structure is formed caused by further coalescence of the carbide inside the alloy. Spheroidised steels represent fairly good machine-ability as the hard carbide particles are embedded within the soft ferrite matrix and thus do not have to be cut with the cutting tool. If the spheroidised steel is heated to immediately above its lower critical temperature the pearlite here will alter to austenite and cooling to room temperature will yield a lamellar pearlite structure plus pro- eutectoid ferrite or cementite depending on carbon content.
Moderating the temperature of tempering via colour appearance is a tradition that is supportive on shop floors. Though, for accuracy the precise temperature measurement are to be preferred. Table explains the colour appearance and temperature in link along with various tools.
Table: Tempering Temperature and Colours of Tools
|
Tool
|
Temperature oC
|
Colour
|
|
Planning tools
|
230
|
Paste straw
|
|
Milling currents
|
240
|
Dark straw
|
|
Taps and dies
|
250
|
Brown
|
|
Punches, drill bits
|
260
|
Purplish-brown
|
|
Press tools
|
270
|
Purple
|
|
Cold chisels
|
280
|
Dark Purple
|
|
Wood saws, springs
|
300
|
Blue
|