Hardening after surface hardening of steels and cast iron
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
High energy heating brings the near surface region (1 to 20mm) to austenitizing temperature. A rapid cooling allows to transform by quenching this region into martensite of high hardness in relation with the carbon content of the alloy.
This hardening is eventually followed by tempering. The hardness gradient obtained is decreasing from the surface. The surface is compressed towards the sub-layer.
CHARACTERIZATION:
Surface hardness (HRC, HRA, HV)
Conventional depth of hard coat Ec or Dc (unless otherwise specified in NFA04-204)
Microstructure requirements
Residual stress rate in compression
APPLICATIONS :
Wear and tear
Fatigue reinforcement
DIFFERENT PROCESSES:
Flame heating
An oxy-gas flashlight (oxy-acetylene, oxy-propane, oxy-methane ...) allows to bring the surface to a high temperature diffusing the heat to the required depth. High depths require a regulation of the heating gradient to avoid surface overheating. The heating is done from near to near or by generalized heating (single-shot).
Induction heating
An inductor crossed by a high frequency current surrounding the zone to be heated or placed near it, develops induced currents which by Joule effect brings the surface to an austenitization temperature.
The terminology distinguishes :
High frequencies between 1MHz and 100 KHz
Medium frequencies between 50Khz and 8 Khz
Low frequencies between 6 and 1KHz
This classification is not normative, the limits set may be different according to the use in the companies.
The choice of frequency is determined by the desired heated depth, the thickness of the induced currents being inversely proportional to the frequency. However, it should be noted that the heated depth is largely achieved by diffusion, but that the choice of a suitable frequency reduces the heating gradient. The heating is done from close to close or by generalized heating (single-shot).
This technique of heating before surface quenching is by far the most used.
Choice of steels
All ferromagnetic steel grades can be heated by induction, it will be necessary to assess their behavior during hardening. The most commonly used steel grades are low to medium hardenability grades with carbon contents between 0.25 and 0.55% (centered at 0.4 - 0.45%) unalloyed or low alloyed.
Choice of fonts
Lamellar or spheroidal graphite gray cast irons with a ferrito-perlite or pearlite matrix (the pearlite must be lamellar) can be hardened by quenching after surface heating. The results obtained are directly related to the amount of combined carbon (in the matrix). These materials commonly hardened by surface quenching must however be subject to precautions relating to the development of the grade (presence of phosphorous eutectic, morphology of the graphite, structure of the matrix...).
APPLICATION OF HARDENING TREATMENTS AFTER SURFACE HARDENING
Hardening by surface hardening, especially by induction (but also more rarely by flashlight, laser...), is widely applied to mechanical parts and is constantly growing.
They are applied for wear and fatigue strengthening.
They concern all branches of mechanics and a very wide range of parts with dimensions ranging from a few mm to several meters in length or diameter, for hardened depths of a few tenths of a mm to 20 mm, subject to an appropriate choice of heating frequencies and hardenability of the steels.
The fields concerned are those :
Automotive and heavy duty industries
Agricultural machinery, handling and public works equipment
Machine tools and industrial equipment
Examples:
In an internal combustion engine: crankshaft, camshaft, tappets, liners, valve stems...
In a gearbox: forks, pins...
In reducers and bridges: shafts, gears, bearings...
In the ground connections of rolling stock: transmission shafts, homokinetic joints, tulips, wheel shafts, shock absorber rods...
In construction machinery: gearboxes, axles, shoes, track train bushings, slewing rings, output shafts, equipment articulation axles, cylinder rods, sprockets (rings and shafts)...
In agricultural machinery: drive shafts, wear parts
In industrial equipment: guide rails, shafts, rolling tracks, rolling mill cylinders, cams, rollers, sedan wheels, machine tool spindles, calibrated rods for the manufacture of cylinders for example (processed in 6 m lengths...)
In tools: saw blades, mower blades, hand tools, stamping and cutting tools...
Implementation
Main equipment (furnace, reactor, line, machine...)
Energy and fluids (gases, chemicals, quenching liquids, salts...)
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