Quenching Hardening After Surface Heating of Steels and Cast Irons
Note: This document reflects industrial experience. The information is provided without any warranty, express or implied, as to its accuracy.
Reviewed by A3TS on July 29, 2026
Principle
A high-energy heating method is used to bring the near-surface region (1 to 20 mm) to the austenitizing temperature. To obtain homogeneous austenite, the heating temperature must be higher than that used for through-hardening (900 to 1100°C). Rapid cooling allows this region to be transformed by quenching into martensite with a high hardness relative to the alloy’s carbon content.
This hardening is eventually followed by tempering. The hardness gradient obtained is decreasing from the surface. The surface is compressed towards the sub-layer.
Surface hardening by quenching following surface heating—particularly by induction or laser, and less commonly by flame or electron beam—is used to harden mechanical parts and is experiencing steady growth.
Characterization:
The surface hardness depends on the carbon content of the selected steel and ranges from 500 to 700 HV. The core hardness remains constant, which helps maintain good impact resistance;
The residual surface stresses are compressive and range from -250 MPa to -700 MPa, which increases resistance to mechanical fatigue.
Treatment depths range from a few tenths of a millimeter to 20 mm, depending on the material, the frequency of the electrical current used, the heating time, the cooling method, and the geometry of the workpiece…;
The conventional depth of the hard layer (Ec or Dc) (unless otherwise specified in NFA04-204) SHD is measured in accordance with the NF EN ISO 18203 standard;
Surface fatigue resistance is greatly increased because the stressed surface layer has a high yield strength.
The deformations after treatment are low because the core is not affected by the treatment;
Benefits
Wear resistance
Fatigue reinforcement
Applications of Hardening Treatments Following Surface Quenching
These processes are used to increase resistance to wear and fatigue. They are applicable to all fields of mechanical engineering and a very wide range of parts, ranging in size from a few millimeters to several meters in length or diameter, with hardened depths ranging from a few tenths of a millimeter to 20 millimeters, provided that the heating frequencies and hardenability of the steels are appropriately selected.
The areas involved are the following:
Mechanical parts of great series.
Gears, pinions and drive shafts ;
Wind turbine bearing ring ;
Rolls of rolling mills.
Limitations
The treatment is suitable for large parts;
Around the areas subjected to high compression, the areas where the profile exits are under high tension. If these areas are subjected to fatigue stress, the part’s endurance limit may be affected;
Very rapid heating and cooling can cause pitting.
Process Parameters
Implementation TECHNOLOGIES (there may be several, or at least variations)
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 through which a high-frequency current flows—whether surrounding the area to be heated or placed near it—generates induced currents that, through the Joule effect, raise the surface to austenitizing temperature.
The terminology distinguishes between:
High frequencies between 1MHz and 100 KHz
Medium frequencies between 50Khz and 8 Khz
Low frequencies between 6 and 1KHz
This classification is not prescriptive; the specified limits may vary depending on how companies use the equipment. The choice of frequency is determined by the desired heating depth, as the thickness of the induced currents is inversely proportional to the frequency. It should be noted, however, that the heating depth is largely achieved through diffusion, but selecting an appropriate frequency reduces the heating gradient. Heating is performed step-by-step or through generalized heating (single-shot).
This technique of heating before surface quenching is by far the most used.
Laser Beam Heating
A laser beam is used in place of a torch, with the advantage—or disadvantage, depending on the situation—of covering a much smaller impact area. In the case of thin sections being heated and sufficiently massive parts, conductive cooling—with the central portions remaining cool—may be sufficient to achieve a cooling rate at least equal to the critical martensitic quenching rate; this is referred to as self-quenching.
Choice of steels
All ferromagnetic steel grades can be heated by induction. The most commonly used steel grades are those with low or medium hardenability, with carbon contents ranging from 0.25 to 0.55% (centered at 0.4–0.45%), and are either unalloyed or low-alloyed. Steels with higher levels of alloying elements (Cr, Mo, V) and higher hardenability are used when greater heat treatment depths are required.
Choice of fonts
Lamellar or spheroidal graphite cast irons with a ferritic-pearlitic or pearlitic matrix (the pearlite must be lamellar) can be hardened by quenching after surface heating. The results obtained are directly related to the amount of carbon dissolved in the matrix. However, when hardening these materials by quenching after surface heating, precautions must be taken regarding the grade’s composition (presence of phosphorus eutectic, graphite morphology, matrix structure, etc.).
Implementation
Main equipment (furnace, reactor, line, machine...)
Energy and fluids (gases, chemicals, quenching liquids, salts...)
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