? Where is your part right now?

Not made yetOne shop: we machine the steel part and heat treat it in the same order, so the geometry, the hardening and the finish grind are planned together rather than across two suppliers. A quench can move a part, so it matters that the allowance is decided before the first cut rather than after.See the processes we run →Already madeSend the finished parts with the steel grade and the hardness the drawing calls for. Heat treatment changes the metal rather than coating it, so tell us which faces have to end up hard and to size before the parts arrive.Check if my parts qualify →Still comparing finishesHeat treatment is not a coating: it adds no layer and no colour, it changes the metal inside the part. If the requirement is a harder steel surface it is the right family - if the requirement is corrosion protection or appearance, it is not.Which route do I need? →

SURFACE FINISHING

Heat Treatment Service

Hardens the steel part Steel parts Not a coating Which route?

Heat treating of the surface is a range of processes used mainly with steel parts that use heating in various ways for different periods to achieve required surface properties. One of the commonly used processes is hardening: the surface of the steel part is first heated to the transformation temperature of 800 degrees C or higher and a certain amount of carbon is dissolved into the surface, then the part is cooled down using oil or water. Prolean offers surface heat treatment for steel parts.

Get a quote Which route do I need?

800 C or higherThe published surface-hardening temperature

Oil or waterThe published quenching media

Dull greyThe published colour of a hardened surface

SteelThe published material family for this process

A steel component during heat treatment, heated to change the structure of the metal rather than to add a coating — Heat treatment is not a coating. It adds no layer and no colour of its own; what the published text describes is a hardened surface on a steel part and the dull grey tone the surface takes after quenching.

Where it fits

Heat treatment runs after machining, and the quench follows the cut

The published route has five steps, and the order matters because the step that produces the hard surface is also the step that can move the part. It is easier to plan around that than to correct it afterwards.

STEP 1Machine the steel partThe cutter sets the geometry. Heat treatment follows machining and it does not correct a dimension that is already final

STEP 2Heat to the transformation temperatureThe surface is heated to 800 degrees C or higher, the published figure for surface hardening

STEP 3Dissolve carbon into the surfaceA certain amount of carbon is dissolved into the surface, which is how a hard case is produced on a steel part. You are here

STEP 4Quench in oil or waterThe part is cooled down using oil or water while carbon is supersaturated in the surface, which produces the hardened surface

STEP 5Inspect the hardened surfaceHardness, distortion and the published dull grey surface colour are checked after quenching

The quench decides the result, and it is also the step that can move the part. Because the change happens inside the metal rather than on top of it, the allowance and the inspection point have to be agreed before the part is cut - see what a quench can move.

A steel part being quenched, cooled rapidly from the transformation temperature

Quenching is the published cooling step: the heated part is cooled down using oil or water while carbon is supersaturated in the surface, which produces the hardened surface.

An illustration of the carburizing process, in which carbon is dissolved into the surface of a heated steel part — Carburizing is the published surface-hardening route: heat the part to 800 degrees C or higher and dissolve a certain amount of carbon into the surface.

An illustration of a steel part during tempering after it has been quenched — Tempering usually follows a through-hardening quench to trade some hardness back for toughness. The parameters we hold for it are being confirmed.

Fit check

Material, section and duty decide whether we heat treat the part

Three things settle it: what the metal is, how its section is shaped, and what the part has to do. The last one is where the route is chosen, and where the wrong route is most often picked.

Material

Published family - the published text says heat treating is used mainly with steel parts, and that is what our surface heat treatment is offered for
Hardenability is set by the grade: the same quench hardens a plain carbon steel and an alloy steel to different depths
Other metals harden by different mechanisms - aluminium alloys gain strength by ageing, not by the carbon-and-quench route
Which steel grades we run, and whether stainless steel is in scope, is being confirmed

The published material is steel. The grades we run, and the other metals, are To confirm.

Section and geometry

A thick section and a thin wall cool at different rates, so the hardened result is not uniform across a part that mixes them
Sharp internal corners and abrupt changes of section concentrate the quench, and that is where distortion and cracking start
Long slender parts and thin plates are the most likely to move during the quench
Tell us the thinnest wall and the heaviest section so the quench is set for the part, not for the average

How far a given part moves is confirmed per job - no distortion figure is published for this process.

What the part has to do

A hard surface over a tough core points to a surface route such as carburizing
A part that must be hard right through its section points to a through-hardening route
A part that must be soft, stable or easier to machine points to annealing or normalizing instead
A part that needs corrosion protection or a colour is not a heat treatment job at all

Specifications

Heat treatment: steel parts, 800 C or higher, oil or water quench

The live page publishes no specification table for this process, so the table below is built from the figures the published text does give. Every line the shop has not confirmed says To confirm rather than a guess.

Process family

Range of processesHeating steel parts in various ways for different periods to achieve required surface properties - as published

Material (as published)

SteelThe published text says heat treating is used mainly with steel parts

Surface-hardening temperature

800 C or higherAs published - the transformation temperature the surface is heated to

Quenching medium

Oil or waterAs published - how the heated part is cooled down

Hardened surface colour

Dull greyAs published - the surface colour of hardened parts

Processes run in house

To confirmWhich of case hardening, through hardening, induction hardening, nitriding, normalizing and stress relief are run in house is being confirmed

Case depth

To confirmNo case depth is published for this process; the depth is confirmed per part rather than quoted

Hardness (HRC)

To confirmNo hardness figure is published for this process; the target is confirmed per material and section

Dimensional change

To confirmA quench can move the part; the limit we hold is confirmed before it is promised against a tolerance

Standards

To confirmNo standard is published for this process

The published figures cover hardening only. The temperature (800 degrees C or higher), the carbon dissolved into the surface, the oil-or-water quench and the dull grey surface colour are the facts published for surface hardening. The other processes in the same family carry no published figures, so every line above that concerns them says To confirm rather than a number.

What it moves

Heat treatment adds no layer, but the quench can move the part

This is where heat treatment differs most from the coating processes. Plating and painting grow the part by the thickness of the layer; heat treatment does the opposite, changing the metal inside the part and adding nothing to the surface. What it can still do is move a dimension, because heating and quenching leave residual stress behind.

Where it acts	What changes	Why it matters	What to do
The surface itself	No layer is added	Unlike plating or painting, no thickness is laid on top; the published change is a hardened surface and its dull grey tone.	Do not add a coating allowance - there is nothing to build up.
Overall size and shape	The part can move or distort	Heating and quenching leave residual stress, and thin sections and sharp changes of section move most.	Agree a distortion limit per part - the figure is To confirm.
Faces that must be hard and to size	A grinding allowance is needed	If a face has to be hard and to size, it is normally ground after hardening, so the cut has to leave material for the grind.	Leave a grinding allowance on any face that carries a tolerance.
Sharp corners and thin walls	Distortion and cracking start here	A corner or a thin wall cools faster than the body around it, which is where a quench does its damage.	Name the critical faces and thicken or round corners where the design allows.
Surface colour	The surface reads dull grey	That is the published colour of a hardened part; it is a result of the process, not a coating or a pigment.	Choose a different process if a colour other than dull grey is wanted.
Corrosion	No corrosion protection is added	A hardened steel surface rusts like any other bare steel surface; heat treatment is not a corrosion barrier.	Specify a coating or passivation if the duty is corrosion.

The direction is clear - heat treatment adds nothing and can move the part - but no distortion figure is published, which is why the dimensional rows above say To confirm. A grinding allowance has to be agreed with the shop, not assumed.

Tell us which faces are ground after hardening and which dimensions are critical, and we will confirm the allowance before the parts are made.

Get a quote

Which route

Carburizing, through hardening, induction or annealing - and the case where the answer is none

The published route is surface hardening, but it is one of a family. Which one applies depends on whether the hardness has to sit at the surface or run through the part, and whether the job is to harden or to soften. The last card is the group where heat treatment is the wrong answer.

An illustration of the case hardening process applied to a steel part, with a hard outer case over a softer core

Case hardening (carburizing)

The published route800 degrees C or higher, then oil or water quench

The surface of the steel part is first heated to the transformation temperature of 800 degrees C or higher and a certain amount of carbon is dissolved into the surface. The part is then cooled down using oil or water while the carbon is supersaturated in the surface, which produces a hardened surface.

It gives a hard outer case over a softer, tougher core, which is why it suits a part that needs wear resistance on the outside without becoming brittle all the way through.

Best for: a hard wearing surface over a tough core on a low-carbon or alloy steel part

Not for: a part that has to be hard through its whole section - that is a through-hardening route.

An illustration of a steel part that has been quenched and then tempered so that it is hard across its whole section

Through hardening (quench and temper)

Hard all the way through

Here the part is heated right through and quenched, so the hardness runs across the whole section rather than sitting in a case. A tempering step usually follows to trade some of the hardness back for toughness.

Best for: a part that must be hard and strong across its full section

Not for: a part that only needs a hard surface - hardening the whole section is more distortion for no benefit. The parameters we hold for it are To confirm.

An illustration of induction hardening, in which a coil heats a local area of a steel part before it is quenched

Induction and flame hardening

A local surface, heated in place

A chosen area - a journal, a gear tooth, a wear face - is heated quickly and quenched, so only that region hardens while the rest of the part is left as it was. It is used where the wear sits in one place and the part cannot be heated all over.

Best for: a defined wear face on a large or otherwise finished part

Not for: a deep case, or a part where the hard zone has to follow a complex shape. See the induction hardening guide.

A diagram of the stages of annealing, from heating through holding to slow cooling

Annealing and normalizing

The opposite: soften the part

Annealing and normalizing are heat treatments too, but the goal is the reverse of hardening: they soften the steel, relieve the stress a cut leaves behind and make the part easier to machine. Annealing cools slowly to soften; normalizing cools in air to refine the grain.

Best for: a part that has to be soft, stable or easier to machine before or after a hardening step

Not for: a part whose duty is wear resistance. See the annealing steel guide.

A chart comparing hardness, strength and stiffness, three different properties of a metal

When heat treatment is the wrong route

The not-applicable case

Heat treatment changes the metal inside a steel part. It adds no layer, no colour and no corrosion barrier, so a job whose real requirement is one of those is not a heat treatment job - and a part whose strength is already sufficient should not be hardened for its own sake.

Consider instead: a plated or coated finish for corrosion and appearance, or anodizing for aluminium

Not for: a part that needs corrosion protection or a colour rather than a harder surface (use passivation, zinc plating or painting), a plastic or non-ferrous part that hardens by another mechanism, or a part that must stay soft and machinable.

Tell us whether the hardness has to sit at the surface or run through the part, and whether the job is to harden or to soften, and we will confirm the route before you commit.

Get a quote

Pick or avoid

When heat treatment is the right call - and when it is not

Pick heat treatment when

The requirement is a harder, more wear resistant steel surface, or a steel part that must be hard and strong across its section.
The material is steel - the published family for this process - with the grade and the other metals confirmed first.
The drawing allows for the small size change a quench can cause, or leaves a grinding allowance on faces that must end up hard and to size.
The part is a steel wear part - a shaft, a gear, a pin, a die - where the surface does the work rather than the looks.
The sequence is planned: rough machine, heat treat, then finish grind, so hardening does not fight the final dimension.
The duty is on the drawing as a hardness or a case depth, so the result can be checked rather than assumed.

Avoid heat treatment when

The job is corrosion protection, a colour or a decorative layer - heat treatment adds none of those. See passivation or zinc plating.
The part must stay soft, ductile or easy to machine - hardening works against that.
The material is plastic, or a non-ferrous metal that hardens by a different mechanism, and the process has not been confirmed for it.
The part is already finished to size with no allowance left, and a quench could move it.
The requirement is stiffness - heat treatment changes hardness and strength, not how much a part deflects under load.
Someone is asking for hardening for its own sake, with no duty on the drawing that needs it.

Not sure whether the part needs a harder surface or a protective one? Send the drawing with the duty on it and we will say which way it should go.

Get a quote

Design constraints

What decides whether the hardened part comes back straight

Six variables: the grade, the shape of the section, the allowance left for the grind, the quench medium, where the step sits in the shop, and what is checked afterwards.

Material and hardenability

The grade decides how deep the quench hardens and how far the part moves. A plain carbon steel and an alloy steel can be given the same treatment and come back with different results, so the grade is the first thing the process is set from.

Section and geometry

A part that mixes thick and thin sections cools unevenly. The thin section can be hardened through and brittle while the thick section is still soft, and the boundary between them is where cracking starts.

Grinding allowance

If a face has to be both hard and to size, it is normally ground after hardening. That means the cut before hardening has to leave a grinding allowance, and the drawing has to say which faces carry it.

The quench medium

The published media are oil and water. Water cools faster and is the harsher quench: it hardens more deeply but distorts and cracks more. Oil is gentler. Which one is right for a given part is set by the material and the section.

Where the step sits in the shop

Heat treatment usually runs between rough and finish machining. Run it on a part that is already to size and there is no allowance left to grind; run it before the geometry is settled and a later cut removes the hardened case.

Inspection after the quench

The part is checked for hardness, for distortion and for the published dull grey surface colour. What can be recorded and supplied with the parts is covered in the QA section below.

Drawing notes

What to write on the drawing so the heat treatment lands where you meant

Two lines carry most of the requirement: whether the part is ground after hardening, and which faces have to be hard and to size.

1.  MATERIAL:  ____________  (STEEL GRADE)2.  HEAT TREAT:  SURFACE  /  THROUGH  (DELETE ONE)3.  CASE DEPTH:  ______  MM  (TO BE CONFIRMED)4.  HARDNESS:  ______  HRC  (TO BE CONFIRMED)5.  QUENCH MEDIUM:  OIL  /  WATER    (AS PUBLISHED FOR SURFACE HARDENING; DELETE ONE)6.  GRINDING ALLOWANCE ON HARDENED FACES:  ______  MM    (TO BE CONFIRMED)7.  DISTORTION LIMIT:  ______  MM  (TO BE CONFIRMED)8.  INSPECT:  HARDNESS, DISTORTION AND SURFACE COLOUR    (DULL GREY IS THE PUBLISHED COLOUR OF A HARDENED SURFACE).

Examples only - replace every blank with your own requirement. The two lines that matter most are the sequence and the allowance: state whether the part is ground after hardening, and leave material on any face that has to be hard and to size.

Which metals

Which metals we heat treat - and the duties that belong to another process

The published text is specific: heat treating is used mainly with steel parts, and our surface heat treatment is offered for steel. The table below shows that, and points the duties that are not heat treatments at the process that handles them.

Material	Heat treatment	Notes	Use instead
Carbon steel	Yes	The published family for this process. A low-carbon grade is the usual choice where a hard case over a tough core is wanted.	—
Alloy steel	Yes	In the published family. The alloying elements change how deep the quench hardens, so the grade is confirmed for each part.	—
Tool steel	To confirm	Commonly hardened and tempered, but no tool steel figure is published for this process - being confirmed.	—
Stainless steel	To confirm	Some stainless grades harden by a precipitation route rather than by carbon and quench. Whether stainless is in scope here is being confirmed.	Precipitation hardening
Aluminium	Different process	Aluminium alloys gain strength by ageing, not by the carbon-and-quench route, so it is a different heat treatment family.	Anodizing for a surface
Plastics	Different process	Heat treatment is a metal process; a polymer part has no microstructure to harden this way.	Confirm the part as a plastic part
Parts needing corrosion protection	Different process	A hardened steel surface rusts like any other bare steel; heat treatment adds no corrosion barrier.	Passivation or zinc plating
Parts needing a colour or a layer	Different process	Heat treatment adds no colour and no layer; the published surface colour is only the dull grey a hardened part takes.	Painting or an anodized finish

Send the drawing with the material and the duty on it - if heat treatment is not the right step for the metal or the requirement, we will say which one is.

Get a quote

Applications

Where a hardened steel surface is doing the work

The parts below are the ones the process is normally bought for: steel parts whose wear is the problem, rather than their appearance.

A 4140 alloy steel part that has been heat treated, its surface reading dull grey

Shafts and gears

A shaft or a gear wears on its bearing surface or its teeth while the rest of the part only has to carry the load. A hard case over a tough core gives the wear surface what it needs without making the whole part brittle.

Case hardened steel fasteners laid out together after heat treatment

Fasteners and hardware

Bolts, pins and studs are case hardened where the threads and the heads have to resist wear and galling, and where the published dull grey surface is acceptable.

A precision nitrided ejector pin, a slender steel part with a hard surface

Pins, dies and tooling

Ejector pins, punches and die details are heat treated for wear resistance and kept straight because the running clearance is small. A slender part like this is also the kind that moves most, so the allowance matters.

Several case hardened steel parts that show a hard wearing surface after treatment

Wear parts and machine components

Cams, wear plates, links and similar components are hardened where the duty is abrasion or repeated contact, and left with the published dull grey surface rather than a decorative finish.

QA and documentation

What you get back with the heat treated parts

The published promise is that parts are inspected and then packed before delivery. Beyond that, here is what we can state today and what is still being confirmed.

Available

Inspection before packing

The published text states that after all parts pass QC inspection they are packed and delivered, so inspection is part of the sequence the parts go through.

To confirm

Hardness record

No hardness figure is published for this process, so whether a measured hardness record can be supplied against an agreed target is being confirmed.

To confirm

Case depth measurement

No case depth is published for this process; whether a measured case depth can be reported on a carburized part is being confirmed.

To confirm

Distortion and dimensional report

Heat treatment can move a part, but no distortion figure is published. Whether a dimensional record can be supplied against an agreed limit is being confirmed.

Where heat treatment sits - and the guides for each process

The published route on this page is hardening. Each process below is part of the same heat treatment family, and each has its own guide.

Case hardening

The process behind the published figures - heating to 800 degrees C or higher, dissolving carbon into the surface and quenching - explained in full.

Induction hardening

How a local area of a part is hardened in place, which suits a defined wear face on a large or otherwise finished component.

Precipitation hardening

The other hardening mechanism, used for some stainless and non-ferrous alloys, where strength comes from ageing rather than from carbon and quench.

Annealing steel guide

The opposite duty: softening steel, relieving the stress a cut leaves behind and improving machinability.

Metal heat treating service

The wider heat treating service, for a part whose requirement is not limited to a hardened surface.

Surface finishing hub

The hub page listing every finish we run, so heat treatment can be seen against the coating, plating and conversion families - which change the surface rather than the metal beneath it.

FAQ

Heat treatment FAQ: the questions engineers ask us

What is heat treatment?

Does heat treatment add a layer like plating?

No. Plating and painting build a layer on the surface; heat treatment changes the metal inside the part and adds nothing on top. That is the boundary between this process and the electroless nickel plating, nickel plating, electroplating and passivation processes on the hub.

What temperature is the part heated to?

For surface hardening the published figure is the transformation temperature of 800 degrees C or higher, after which a certain amount of carbon is dissolved into the surface. Other processes in the family carry no published temperature, so those are confirmed per job.

How is the part cooled?

By quenching - the published media are oil or water. The part is cooled down while carbon is supersaturated in the surface, which produces the hardened surface.

Will heat treatment change my dimensions?

It can. Heat treatment adds no layer, so there is no coating thickness to allow for - but heating and quenching leave residual stress and can distort the part, most of all where the section is thin or changes sharply. The distortion figure is being confirmed, so a grinding allowance is agreed for faces that must end up hard and to size. See what a quench can move.

Which materials can be heat treated?

The published text says it is used mainly with steel parts, and that is what our surface heat treatment is offered for. Which steel grades and whether stainless steel is in scope is being confirmed. See the material table.

What does a hardened surface look like?

The published text states that the surface colour of hardened parts changes to a dull grey. That is a result of the process, not a coating or a pigment, so a part that needs a different colour is a different job.

Is heat treatment the same as a surface finish?

It sits in the same family on our hub, but the mechanism is different: a surface finish changes the surface, while heat treatment changes the metal beneath it. A part that needs a harder surface is a heat treatment job; a part that needs corrosion protection or a colour is not.

How do I get a quote?

Send the drawing with the material, the duty and the faces that have to be hard. The published promise is a free quote from a real engineer, as fast as one hour, and in general an evaluation and a quotation within six hours. See how ordering works.

Get your heat treatment quote

Upload a STEP, IGES or PDF drawing. Tell us the material, the duty - wear, toughness or a hardness target - and whether the hardness has to sit at the surface or run through the part, and we will confirm the route, the allowance and the sequence.

Get a quote Machine it first

What to send us

Drawing - 2D and 3D files (STEP, IGES or PDF).
Material - the steel grade, so hardenability and quench can be set from it.
Duty - what the part has to resist, and any hardness or case depth on the drawing.
Faces to grind - which surfaces are ground after hardening, so the allowance is left.
Critical dimensions - any size that may not move during the quench.
What happens next - the assembly or the next operation, so the hand-over state fits.

The published sequence is: get a quote as fast as one hour, start production once the order is confirmed, then receive the part after it passes inspection and is packed. All uploads are secure and confidential.