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Most common heat treatment defects ,Causes

&

Remedies:

Heat Treatment and Material Testing

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Insufficient

hardnessCauses: Decarburization

Remedies:

Sufficient machining stock (pre-existing decarburization).

Heat the tools protected by appropriate antidecarburizing

media or heat in a salt bath, protective atmosphere,

possibly in a vacuum furnace.

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Insufficient

hardnessCauses: Low hardening temperature or too

fast, uneven heating

Remedies:

Check the control instruments. Where possible,

anneal or repeat the hardening treatment.

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Insufficient

hardnessCauses: Insufficient soaking at hardening

temperature

Remedies:

The holding times at hardening temperature

refer to the condition in which the temperature has

been reached evenly throughout the entire section(need for preheating). Where possible, anneal and

repeat the hardening treatment.

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Insufficient

hardnessCauses: Cooling rate:

a) unsuitable cooling medium for the grade of steel;

b) insufficient cooling capacity, air not sufficiently

ventilated;

c) parts covered by films that form insulating layers.

Remedies:

Check the prescriptions of the steel supplier. The relationship

between the volume of the cooling medium and the tool mustbe at least 5 : 1 with cooling recirculation. In the case ofcooling in air, increase ventilation. Avoid formation of oxide,layers of steam (soft areas).

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Insufficient

hardnessCauses: Tempering temperature too high.

Remedies:

Anneal the tools and repeat the heat treatment

controlling the tempering temperature.

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Uneven hardnessCauses: Formation of bubbles of steam during

cooling pack hardened tools scale patch decarburiz. imperfect cleaning of the

surfaces.

Remedies:

Stirring of the bath.Appropriate dipping method.

Clean the parts thoroughly before heating.

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Excess surface

hardnessCauses: During hardening heating, a carburizing

and embrittlement effect occurs, surface

cracks

Remedies:

Check the activity of the anti-decarburization

media (they must not be carburizing) and of theinoculants of the salt bath or of the protective

atmospheres.

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BrittlenessCauses: Excessive hardness-insufficient

tempering.

Overheating (coarse fracture and bright

grain).Remedies:

Increase the temperature and tempering holding

time.Harden at lower temperature reduce holding timeat tempering temperature.

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DeformationsCauses: Uneven heating.Too fast cooling.

Part incorrectly supported in furnace.

Incorrect dipping in quenching bath.

Stresses present before heat treatment.Remedies:

Preheat check furnace capacity.

Change to step quenching reduce the hardening temperature.

Check the supports

if possible heat vertically.Review method of dipping tools during cooling.

Perform suitable stress relieving after rough machining.

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Fractures during CoolingCauses: Uneven heating

Too fast cooling.

Overheating.

Remedies:

Preheat.

Select the cooling medium according to the shapeof the tool and hardenability of the steel.

Reduce the hardening temperature.

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Fractures during

quenching cooling

(before tempering)Causes: Incomplete transformations that continuewith the tool cold.

Retained austenite.

Keep the non-tempered part in a coldenvironment.

Remedies:

Temper immediately after hardening.Where possible, carry out a suitable treatment tocomplete the transformation.

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Cracks after

temperingCauses:

Insertion of the tools in the furnace without

preheating for tempering.

Remedies:

For tempering at high temperatures, preheat

the tools to 200 - 300 C