HARDENING

This means the increase of hardness resp. strength by means of a thermal treatment. This treatment consists of the steps austenising and quenching. Austenising shows an annealing at 800-1.200° C,whereby a face-centred cubic crystal lattice follows.By the subsequent quenching a microstructural transformation results, which creates a tensed, hard martensite-lattice. The parameters (austenising temperature, duration and critical speed of quenching) depend on the composition of the steel. After the hardening normally a tempering follows to increase the toughness.

Hardening and tempering in protective atmosphere

At hardening in a protective atmosphere the unwanted internal oxidation can be avoided by means of a controlled gaseous atmosphere. The quenching happens in oil or polymer solution.

Today vacuum heat treatment of tools and parts is state of the art. Various heat treating methods like tempering, hardening and annealing can be processed.

Vacuum hardening is defined as heating of parts in a sealed tank. The heating up to 700° C is performed in a circulating nitrogen atmosphere. Afterwards a vakuum is made through pumping down the gas. Electrical heated graphite bars warm the batch up to the right temperature. Quenching is done by injection of gaseous nitrogen at a pressure of up to 10 bar.

Advantages:

– Low distortion
– No decarburisation
– No oxidation
– Blank surfaces
– Documented process
– Repeatable results

Case hardening consists of carburising and quenching. When carburising the surface of a low-carbon steel gets enriched with carbon (temperature 880-940 °C). When quenching from hardening temperature in an oil bath afterwards a hardness profile according to the carbon content results.

Carburising is a diffusion process and lasts several hours according to the required case hardening depth. After carburising the materials are quenched and the martensitic outer zone is produced. As during conventional hardening a tempering after the carburising has to be done. The result of the treatment is a combination of a hard outer zone and a ductile core. Carbonitriding is a modification of case hardening.

For carburising batch furnace or pot furnace constructions and cycle constructions are used. Normally the process is controlled fully automatic and adjusted according to the reqirements like case hardening depth, outer zone- C-content etc.

Carbonitriding is a variant of case hardening, this means an enrichment of the surface layer with carbon and nitrogen. Like case hardening the carbonitriding produces wear-resistant surfaces and increases the endurance strength of dynamic loaded components. The process temperature of 850-900 °C is slightly lower than at case hardening. Along with carbon the surface layer is enriched with nitrogen during carbonitriding. Hereby the hardenability of the surface layer improves, whereby also unalloyed steels can be treated, at which during case hardening the danger of soft spots may be.

Annealing is a preamble of a multiplicity of different annealing processes. Annealing is used to set certain mechanical or physical characteristics in the material. Generally the strength of the material will not be increased through an annealing process.

Essential annealing processes are:

– Normalising
– Stress relieving
– Softening

Induction hardening is among surface-layer hardening processes. This means in the near-surface peripheral zone of the part it gets down to a martensitic hardening and thereby a hardness increment in this zone through a controlled cycle of heating / quenching. Structure and hardness in the core of the part remain uninfluenced.

Hardening and tempering in protective atmosphere:

At hardening in a protective atmosphere the unwanted internal oxidation can be avoided by means of a controlled gaseous atmosphere. The quenching happens in oil or polymer solution.

Induction hardening consists of the following steps:

– Heating the surface to hardening temperature.
– Quenching in water, oil or polymer solution.
– Tempering to increase toughness if necessary.

Advantages:
– Application-oriented local hardening profile achievable.
– Fine hardening structure because of the quick heating-up rate, thereby high toughness.
– Low distortion low due to local heating.
– Energy-efficient process with high Efficiency.
– Wear protection on stressed spots.
– Improvement of endurance limit, as compressive stresses occur in the surface

Tenifer + QPQ designates the classical salt bath nitrocarburizing and is an option of the thermochemical edge layer procedure “nitrocarburizing”. As with nitrocarburizing in gas or plasma the wear resistance and endurance limit of the parts will be increased.
The QPQ – procedure (quench–polish–quench) additionally generates a corrosion resistance on the surface of the part through the combination Tenifer + polishing of the parts + oxidation. The treatment temperature for nitrocarburizing in melted salt is at 580 °C.

Tenifer®: registered trademark of Houghton Durferrit GmbH

Advantages:
– Improvement of friction and sliding properties
– High wear resistance at abrasion and adhesion
– Good endurance limit
– Low size alteration or deformation
– Applicable for distortion endangered parts

Solution annealing with subsequent warm-ageing increases the strength of the material through precipitation-hardening. Basically precipitation-hardening encompasses all heat treatment measures that utilise the temperature-dependency of the solubility of mixed crystals to obtain an increase in strength.

Age hardening comprises three stages:
– Solution annealing
– Quenching
– Ageing (hot or cold)

Solution annealing is carried out, depending on the alloy, at temperatures between 480–550 °C.
A temperature is selected at which a sufficient amount of alloying elements is dissolved in the mixed crystals to ensure that the precipitation-hardening effect occurs after quenching and ageing.

The precipitation-hardening effect depends on the solution annealing temperature, pre-annealing time, temperature of the cooling medium, temperature of the component at the moment of quenching, ageing temperature and ageing time.