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Soft annealing Soft annealing is carried out at a temperature of just under Ac1*, sometimes also over Ac1 or 

 by fluctuating around Ac1 with subsequent slow cooling to achieve a soft condition (DIN 17022

part 1-5). Through this heat treatment, the cementite lamination of the perlite is transformed to a

spherical form - known as granular cementite. This type of microstructure provides the best

workability for steels with a C-content of more than approx. 0.5%. Granular cementite providesthe condition for best workability for any type of cold working e.g. for cold-heading, drawing, or cold extrusion.

In practice, the steel is heated to the prescribed tempering temperature, maintained at this

temperature for some time and then slowly cooled in the furnace to approx. 600°C before beingcooled in air. Through hardening followed by soft annealing, it is possible to achieve a

 particularly even annealed microstructure with finely distributed granular cementite.

Normalising 

When normalising, the steel is heated to a temperature (approx. 20°C to 50°C) above theupper transformation point Ac3*), for hypereutectoid steels above Ac1, and is then cooled in

static air. It is used to achieve an even, fine-grained microstructure.

For hypoeutectoid steels, the microstructure consists of perlite and ferrite and for hypereutectoidsteels of perlite and cementite. Grain refining is carried out by going through the a-g -

transformation *) twice, during heating and cooling. The higher the heating and cooling speeds,

the finer the grains in the microstrurture become, providing that the transformation duringcooling takes place in the perlite stage.

Through normalising, an uneven and coarse grained microstructure which has come about during

hot forming can be eliminated. In addition, for steels with C-contents of less than 0.5% which

transform easily, the adjustment to a perlitic-ferritic-microstructure with largely even distributionleads to good machining properties.

Air cooling can only be used for steels which transform fully in the perlite stage. This is true of 

non-alloyed and low alloy steels. In case of doubt, information in this regard can be found int the

time-temperature diagram for continuous cooling along with Figure 12. For higher alloy steels,isothermal perlite transformation is effective. Based on the time-temperature diagram for 

isothermal transformation and allowing an adequate safety margin for the time period, the

temperature where the most rapid formation of perlite occurs is selected.

Hardening (Quench hardening) 

The term hardening is used to describe cooling from a temperature above the transformation points A3 or A1 at such a speed that on the surface or throughout, there is a significant increase

in hardness, generally through the formation of martensite. The heating must be carried out to atemperature above the transformation points Ac1 or Ac3 and the cooling from a temperature 

above the transformation points Ac1 or Ac3 (DIN17022 Part 1-5).

The aim of the hardening is to achieve as high a level of hardenss as possible in the workpiece.

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The hardness reached depends on the carbon content of the steel and its hardenability whereby

the dimensions of the workpiece and conditions during heat treatment also play a role. In order to

carry out optimum hardening, it is necessary to adhere to the temperatures given and times for holding these as well as to correctly select and handle the hardening medium. The most suitable

hardening values are to be achieved through hardening during the martensite phase.

Ferrite in the hardened microstructure is caused by a hardening temperature which was too low

or is a result of cooling too slowly. Residual austenite can occur in high carbon alloyed steels if  the hardening temperature was too high. In these cases there is ususally also coarsening of the

grains. If the proportion of ferrite, perlite or bainite is too high, there is a reduction in hardness

and the toughness properties are also diminished.

The quenching media are usually water, oil or air, whereby the application depends on the

critical cooling spped of the steel. In each case, the mildest quenching medium possible is usedfor each particular case, in order to keep the risk of tearing and distortion to a minimum.

*) Definitions:A1 point = eutechtoid transformation (723°)

A3 point = (a-g )-iron-transformationAc1,Ac3 point = Arresting point on the heating curve at A1- a/o. A3- transformation

(c = chauffage)

Ar 1,Ar 3 point = Arresting point on the cooling curve at A1- a/o. A3-transformation

(r = refroidissement)

Warm bath hardening 

The term warm bath hardening is used to describe hardening of a workpiece by cooling in a saltor metal bath with retardation until temperature equalisation is achieved and then subsequently

cooling as required to room temperature (DIN 17022 part 1-5). This treatment is used when

there is a risk of distortion because of the shape of the part. One prerequisite is that the steel 

used can be hardened in oil. A transformation gap between perlite and bainite or sufficient timefor starting in the bainite range above the MS temperature are desired characteristics for the

 particular isothermal transformations.

Austempering 

The term austempering is used to describe the quenching of a workpiece from the hardening

temperature in salt and metal baths of a temperature lower than is required for the formation

of perlite but higher than for the formation of martensite. This is maintained until the

transformation to bainite has ended and there is subsequent cooling as desired to room

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temperature (DIN 17022 Teil 1-5). Isothermal transformation into bainite of this type results in

very low distortion levels and excellent toughness properties. Tempering is not required.

Tempering 

The term tempering is used to describe heating after previously hardening, cold working (cold

levelling) or welding to a temperature between room temperature and below the

transformation point Ac1 and holding at this temperature with subsequent cooling as suits the purpose (DIN 17022 part 1-5).

A microstructure which has been transformed quickly through rapid cooling is not in a stable

state of equilibrium meaning that when reheated, the toughness increases and, at the same time,

the hardness can be decreased. The amount by which the hardness decreases is determined by the

temperature and time period for tempering.

The decrease in hardness takes place in various steps which are characterized by certain

 precipitation and transformation processes.

First, the tetragonal martensite is tranformed (at approx. 200 ° C) into cubic martensite which isless prone to tearing and above this temperature more and more carbides are gradually

 precipitated. If there is residual austenite in higher alloy steels, these precipitation processes

cause a lack of carbon in the residual austenite which hence converts to tetragonal matensiteduring cooling. In such cases, additional tempering is required.

The tempering treatment should be carried out immediately after hardening in order to prevent

tearing through tension. The adjustment of the mechanical values is detemined to a far greater degree by the temperature during tempering than by the time. Generally, a temperature 

holidng period of one hour is selected per 25 mm wall thickness. The cooling after tempering

depends on the shape of the workpiece and on the quality of steel. For more complicated pieces,cooling too rapidly can casue unnacceptable levels of strain.

With slower cooling from the tempering temperature, tempering brittleness can occur which becomes apparent in the decrease in impact strength. This embrittlement occurs mostly in allyo

steels containing Mn, CrMn and CrNi when, after tempering, the temperature range 550 to

400°C is slowly passed through or the temperature is held in this range for a long period. The

tendency towards tempering brittleness can be weakened by adding Mo to these steels. It is also possible that, through cooling with a greater thermal shock, the tempering brittleness can be

suppressed if tempering has been carried out above the risk temperature, i.e. above 550°C.

The changes in mechanical characteristics depending on the tempering temperature lead to aconstant reduction in hardness without any noticeable increase in toughness up to a temperature 

of approx. 500°C. It is only at above 500°C that there is a strong increase in toughness with a

further decrease in hardness.

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Stress Relieve Annealing 

The term stress relieve annealing is used to describe annealing at a temperature below the

transformation point Ac1, mostly at under 600°C followed by slow cooling to release internal

stresses without bringing about any significant changes in the exisiting properties (DIN 17014).

It is used where, due to inherent stresses, distortion or tearing of the workpiece can occur. Thestresses can have arisen as a result of the increase in volume of the crystal lattice (e.g. with theformation of martensite), due to irregular temperature changes or as a result of cold working

(levelling processes).

Generally, temperatures of between 450 and 650 °C are used. The temperature should certainly

remain 30-50°C below the tempering temperature.

The decrease in the flow limit with increasing temperature leads to a reduction in the stresses

and this becomes apparent in corresponding distortion. For this reason, stress releif can best beachieved if the workpiece is heated above the transformation level, allowed to cool slowly and

the distortion which has occurred is then worked off. Appropriately overdimensioning isnecessary for this method of working. Stress relief in pieces which are to retain a high level of 

hardness (e.g. case hardened pieces) is carried out at approx. 200°C. The reduction in stress is

achieved through the transformation of tetragonal martensit into cubic martensite which has less

tension.

OB Annealing (a term introduced by us) 

We use the term OB annealing to describe heat treatment for adjusting the microstructure toallow optimum workablitiy (condition BG*).

The microstructure which is most suitable for the workability can be very varied and depends onthe type of machining, the chemical composition and the behaviour of the steels during

transformation. Whereas the best properties for machining purposes are demonstrated in steels

with low and medium C contents and lower alloy content and which have a microstructureconsisting of lamellar perlite and ferrit, for steels with more carbon and higher alloy content, it is

advantageious to have a microstructure containing granular cementite. In order to adapt the steel 

to optimum workability, the type of processing used is of essential importance since the same

microstrucutre can behave differently in various processes. For carrying out OB-annealing, thesame regulations apply as described for normalising, tempering and soft annealing, whereby the

treatment can be carried out with continuous or with isothermal transformation.

Full annealing (Coarse grain annealing) 

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The term full annealing is used to describe annealing at a temperature above the upper 

transformation point Ac3 with cooling as required to suit the purpose and achieve a coarser 

grain.

As a result of the coarse grains, good workability is obtained, above all, in steels with a low

carbon content and a highly ferritic-perlitic microstructure. This improvement is based on thefact that the workpiece with coarse grains has a low degree of toughness meaning that a slightly

 brittle swarf occurs on it and this, in turn, leads to a reduction in wear when cutting.

Case hardening 

The term case hardening is used to describe hardening after previous carburization and, possibly,

simultaneously increasing the nitrogen content of the surface (DIN 17014).

It is used in cases where,besides high core toughness, a workpiece also needs to have a hard

surface which is resistent to wear. Furthermore, through case hardening, the fatigue strength isincreased at the edges due to inherent stresses. Steels used for this have low C contents and,depending on the desired core toughness, may be alloyed.

Carburization ( case hardening, cementing) is the enrichening with carbon in an area limited tothe edges by holding the temperature above the transformation points Ac1, or Ac3 in carbonreleasing media. Depending on the type of medium used, it is described as gas, bath, powder or 

 paste carburization (DIN 17014).

The carburization depth is determined by the length of time required for cementation and the

activity of the carburization medium. Generally, the case hardening temperatures lie at about

870-930°C but sometimes higher temperatures are used.

For case hardening, depending on the material and shape and size of the workpieces, various

types of treatment can be considered of which some are shown in the form of a diagram in figure

20. 

Figure 20 (to enlarge please klick to figure in question)

Examples of successful treatment in case hardening steels

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*) Heat treatment to achieve a particular microstructural formation (BG): The workpieces arecooled in a controlled process from a temperature of between 900 and 1000°C.

1. Direct hardening (hardening out of the case) involves hardening the carburized workpiece at

the end of the carburization process whereby the hardening temperature can be lower than the

temperature during carburization but must be higher than, Ar1 the case (DIN 17014).

2. Single hardening (case refining) involves case hardening of the carburized workpiece and,after that, cooling to below the transformation point Ar1 abgekühlten from the hardening

temperature of the (DIN 17014).

3. Double hardening involves hardening the carburized workpiece twice whereby the first

hardening is carried out from the the hardening temperature of the core workpiece, the second

from the hardening temperature of the case (DIN 17014).

For both single and double hardening, intermediate annealing of the carburized components can

 be carried out before the edge hardening. This heat treatment consist of annealing at just under 

Ac1, i.e. mostly at approx. 600-650°C with a longer holding time and subsequent slow cooling.

Due to this intermediate anealing, it is possible to cut out the carbon released as surplus in theaustenite carbon as cementite and avoid the risk of formation of residual austenite during the

subsequent case refining process. It also results in a reduction in distortion.

Tempering The term tempering is used to describe heat treatment to achieve high levels of toughness with a

 particular tensile strength by hardening and subsequently annealing, normally at hightemperatures(DIN 17014).

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The mechanical properties of a tempered steel, in particular its toughness, depend to a large

degree on the care taken during the tempering treatment.

The best limit of elasticity ratio and greatest toughness are achieved in tempering when full

hardening has been carried out above the martensite stage. Suitable properties can still beachieved when tempering with large cross sections, if , after hardening, a microstructureconsisting of at least 50% martensite is achieved in the core.

In order to achieve the desired properties, the tempering diagrams whch are given for most typesog steel can be used. The selection of steels for a desired degree of hardness is also based on the

tempering cross-sections which are dealt with in the information regarding mechanical properties

(DIN 17.200).

Surface hardening 

The term surface hardening is used to describe heating of workpieces which is confined to thesurface during which the core remains below the hardening temperature and is not hardened at

all during quenching. This heating confined to the surface is achieved by gas flames (flamehardening) or inductive heating (induction hardening). As a result of these types of heating,

under corresponding conditions, it is possible to achieve heating to hardening temperature 

throughout but then these types of heating can no longer be called surface hardening. Special

types of surface hardening are case hardening and nitride hardening.

Surface hardening is used where a hard and wear-resistent surface with tough core properties isrequired. At the same time, the fatigue strength is improved. The C contents of the steels which

can be used are to be adapted to the desired surface hardness (see DIN 17.212). The surfacehardness which can be achieved depends on the C content of the workpiece, the hardening depthdepends on the alloy basis and the hardening conditions and, in particular, on the austenitising

temperature (which is higher than for normal hardening), as well as on the heating period and

austenitisation period. In practice, these factors are controlled by the energy input per time unitand the feeding speed.

Immediately after hardening, the parts are treated at approx. 200°C to relieve stresses (see also

under "Steels for Surface Hardening").

Nitriding (Gas and Salt Bath Nitriding) 

 Nitriding consists of annealing in nitrogen releasing media to achieve a nitrogen-enriched surface(DIN 17014).

The nitrided surface reaches a high level of hardness, good resistence to wear and annealingstability up to 500°C - providing there is a tendency for special nitrides to form.

 Nitriding can be carried out in an ammoniac gas flow at 500°C; the nitriding period amounts to

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approx. 10 to 100 hours. In addition, nitriding is carried out in salt baths where temperatures are

at 550 to 580°C and this treatment period is significantly shorter.

The depth of hardening can reach up to 1 mm if the period of nitriding in gas is suficiently long.

In a salt bath it is a few tenths of mm Whereas gas nitriding is mostly used for nitriding steelswhich can achieve very high levels of hardness because of their Al and Cr contents, in salt bath

 processes all steels are nitrided. For these steels without nitride forming elements, the nitriding

 process in a salt bath (soft nitriding) only leads to a small increase in the surface hardness.Resistence to wear, however, is greatly increased.

For the nitriding treatment, it is necessary to use material (DIN 17.211) which has sufficiently

low-stess levels and which has had its surface cleaned and all grease removed. Furthermore,there must be no sharp transitional edges. The most suitable properties of the nitriding layer are

achieved after tempering as the microstructure is then even and has no spots of ferrite.

Decarburized edges must be avoided. For gas nitriding, cooling is carried out slowly in the

furnace so that, after cooling, a workpiece which is largely free of distortion is achieved.

Tenifer treatment 

Tenifer treatment is a salt bath process especially developed from soft nitriding. As a nitrogen

carrier, a KCN/KCNO salt bath with air cooling is used. The parts are treated at approx.. 570°C for between 30 and 120 min. and are then cooled in water or air, depending on the material and

shape.

The surface which has been treated in this way, consists of two layers, a so-called connecting

zone and, beneath that, a diffusion zone. The former consists of carbon nitrides and is betwee 12

and 16µ thick. The latter contains released nitrogen which precipitates and forms needle-shapednitrides only during slow cooling or annealing at over 300°C. In alloy steels, special nitrides

form in the diffusion zone. Both zones are more than 0.6 mm thick .

Tenifer treatement has proved itself in particular for the treatment of parts which are subjected to

sliding friction (e.g. at bearings). The diffusion zone means that the fatigue strength is also

increased. The improved sliding properties result from the improved friction coefficients in the

connecting zone meaning that seizing is prevented. Tenifer treated parts also have a certainresistence to corrosion.

Carbonitriding 

The term carbonitriding is used to described simultaneous enrichment with carbon and nitrogen

at the area round the edge by holding the temperature above or, if need be, also below thetransformation point Ac1 of the core material in carbon and nitrogen releasing media (DIN

17014). Finally, depending on the properties required, cooling is carried out in water, oil or air.

Carbonitrided workpieces are more resistent to wear than case hardened ones but the core

hardness is lower than with case hardening.

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The treatment can be carried out in a gas flow or in a salt bath at lower temperatures than for 

case hardening (700°C to 800°C). There is therefore less thermal shock during quenching. As aresult of the presence of nitrogen in the layer at the edge, the critical cooling speed is reduced.

For this reason, there is less distortion in the parts.

Depending on the temperature guidance and various proportions of nitrogen carriers in the thegas or bath, the composition of the edge zone can vary. With a low treatment temperature, there

is first a carbonitride layer consisting of carbonitrides and cementite and, below this, due to the

high nitrogen content of the layer, there is a hardened microstructure. At higher treatmenttempertures, the carbonitride layer consists of cementite in which the carbon can be replaced by

nitrogen.

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