Understanding and Specifying Steel Heat Treating
For anyone who is interested. Design, manufacturing, testing and tooling people. A crash course
covering basic metallurgical theory and some practical advice on what you need to do before
specifying heat treating for mechanical components. How to negotiate testing of production lots.
OUTLINE OF STEEL HEAT TREATING SEMINAR-
- WHAT IS STEEL
- CHEMISTRY OF STEEL
- Steel is mainly iron (usually 90-99+%) with small amounts of
- carbon (__)
- manganese (__)
- and sometimes others...
- TYPES OF STEEL (1010, 4340, D2, etc.)
- There are two ways to divide steel
- Carbon content being High, Medium or Low
- Alloyed or not
- 4 Digit codes system adopted by SAE and AISI uses first two digits to
identify family of steel, last two digits tell amount of carbon.
- We'll do amount of carbon first. For example, in 1010 steel, the
second ten means that there is 10/100% carbon or .10%
carbon, (approximately) in that grade. In 4340 steel, there is
40/100% or .40% carbon.
- The first digit tells us whether there are any elements which are
considered "alloys" in it. (Alloys are the elements listed in A.1.
above BELOW the letter "e".)
- If the first digit is a "1", there are NO alloys.
- If the first digit is not a "1", there are some alloys.
- The second digit tells us
- NOTHING if the first digit is not a "1".
- Whether the material is capable of being "good", tough,
crack resistant steel if the first digit is a "1". In 1nxx
series steels, if the "n" is a
- "0", then it is plain, possibly good carbon
- "1", then it is resulfurized steel which is good for
machining and can be highly crack prone in
- "2", then it is resulfurized and rephosphorized,
which is also good for machining, and can also
be very highly crack prone in some applications.
- "3" or "5", it has extra manganese, which can
make it easier to harden a larger or more
intricately shaped piece without cracking.
- Tool steels are shown by a letter and a number. The number has no
particular significance. The letters have the following meanings:
- Cold Work Tool Steels (stamping dies, punches, etc.)
- W grades must be Water quenched in order to harden
and are the most prone to cracking, but are the least
expensive of the tool steels.
- O grades are hardened by quenching in oil, and are
somewhat less prone to cracking
- A and D grades are both hardened by cooling in air, and
are the least crack prone, and the most expensive.
- Hot Work Steels (extrusion dies) start with "H"
- High Speed Steels (high speed drill bits, etc.) start with "T" or
"M" for tungsten alloy or molybdenum alloy
- Shock Resisting Steels start with S
- Special purpose steels with lower carbon start with L
- No information about the composition can be obtained by
looking at the grade name in tool steels, You must look it up!
- Each grade has a relatively wide range of properties to which it may be
processed. The particular application will cause the range to be
- Solid steel is a metallic, crystalline substance.
ATOMIC ARRANGEMENT (MARSHMALLOW AND TOOTHPICK MODELS)
- Iron atoms are in a cubic structure, which is so small, we can only see
it in the most powerful modern electron microscopes
- The arrangement of atoms is "Body Centered Cubic" at room
temperature, and changes to "Face Centered Cubic at high
- Carbon fits into the "empty" spaces between the iron atoms.
- Many of the alloying elements substitute for an iron atom in the crystal
- Most commercial steel is polycrystalline. We call individual crystals in metals
- Grains are individual crystals which are made up of many, many
atoms. Grains are visible in a regular microscope and sometimes by
- The reason grains don't look as regular as say, salt crystals, is that
they form as the steel is solidifying, and as the individual grains grow,
they run into each other and form somewhat irregular shapes.
- Fine grains are small, and cause the steel to be crack resistant in
impact and cold temperature situations. They also provide good
surface finish in heavily drawn sheet metal.
- Coarse grains are larger, and cause the steel to have good resistance
to "creep",or slow shape change leading to cracks, at high
temperatures. Medium coarse grain steel is the most ductile and
- WHAT IS HARDNESS AND WHY DO WE CARE HOW HARD THE METAL IS?
- Hardness is generally defined in metals applications as resistance to
- Hardness is an important property because it is EASY to check and it has
been shown to correlate well with many other properties, including strength
and wear resistance.
- The common hardness scales in order of usefulness on hefty hunks to thin
tiny pieces or surfaces of surface hardened parts are
- Rockwell superficial
- Microhardness (Knoop or Vickers)
- The different hardness scales can be compared for steels using a
hardness conversion chart.
- The desired hardness range for a given metal depends on its composition
- Higher carbon steels or steels which have had carbon added to their
surfaces (carburized) can safely be used at higher hardness ranges
than lower carbon steels.
- High carbon steels may be used in the range of HRC 50-65
- Medium carbons steels may be used up to 50 HRC
- Low carbon steels should be used in the HRB range, unless
they are carburized.
- If carburized, the surface hardness should be based on
the surface carbon content.
- The application of a particular part also influences the maximum
hardness which should be specified. For shock or impact loading,
even lower hardness values may be necessary to prevent unexpected
- Sources of recommended hardness ranges:
- Metals Handbook, 9th Ed, Vols 1,3 and 4
- Heat Treaters Guide
- WHAT IS HEAT TREATING
- TYPES OF HEAT TREATING
- Heat to cherry red, cool "quickly".
- Heat to a moderate temperature, cool in air, purpose to reduce
brittleness and residual stress of as hardened parts, also
- Heat to cherry red, cool in air.
- Purpose to refine grain structure, make steel more uniform
from a microscopic view, make steel have more uniform
mechanical properties, sometimes used as a stress relief.
- Heat to cherry red, cool in furnace (very slowly).
- Purpose to soften, improve machinability, formability,
sometimes to control magnetic properties.
- STRESS RELIEVING
- Heat to a moderate temperature, cool in air
- Purpose to relieve stresses from operations such as stamping,
welding, heavy machining, etc.
- CASE HARDENING (CARBURIZING, INDUCTION HARDENING,
- Purpose to harden the surface to improve wear resistance,
while leaving a soft, crack resistant core.
- Carburizing: Heat to cherry red in a carbon rich atmosphere,
and quench in water or oil. Actual carbon content at the surface
is increased. Best done on low carbon steel.
- Induction Hardening: Heat a medium carbon steel at the
surface only by use of an electricity carrying coil surrounding
the part which induces electric current in the part which causes
the part to heat up VERY FAST. The part is then quenched in
water or oil. Carbon content at the surface is unchanged by this
- Flame Hardening: Same as induction hardening, except heat is
provided with a flame. This method is more flexible for large or
odd shaped parts which are difficult or expensive to design
induction coils for. If done by hand, variation from part to part
may be pretty large.
- HOW DOES HEAT TREATING CHANGE THE PROPERTIES OF THE STEEL?
- ATOMIC ARRANGEMENT CHANGES (MORE MARSHMALLOW
- CARBON SOLUBILITY CHANGES
- As a function of temperature
- As a function of atomic arrangement
- THERMAL EXPANSION AND CONTRACTION
- BAD CHANGES
- DISTORTION is due to uneven heat extraction rates during cooling
from cherry red. This can be because of thick and thin sections in the
same part, irregular shapes that trap vapor, water in oil,) as well as
relief of prior residual stresses.
- CRACKS are what happens when distortion becomes excessive, or if
the cooling rate is too fast for the size and grade of steel, or if there
are imperfections in the steel.
- GRAIN COARSENING happens when the material stays at the cherry
red temperature too long. Grain size is an important factor in
determining the minimum cooling rate required for hardening. Coarse
grains are BAD for many applications.
- BASIC HEAT TREATING THEORY
- HOW HOT IS HOT? (HEAT TREATING TEMPERATURES)
- Determining heat treating temperatures from the iron carbon "Phase
Diagram" (Composition vs. Temp)
- Hardening: Close to, but above 100% fcc line*
- Normalizing: 100 degrees above 100% fcc line*
- Annealing: Many different ways
- Tempering: BELOW EUTECTOID
* For some very high carbon steels, may be at a lower temp. (D2)
- HOW FAST IS FAST? (COOLING RATES FOR HARDENING) (Time vs
- Use of Continuous Cooling Curves
- EFFECT OF SECTION SIZE
- What Does Hardened Mean?
- Ideal Critical Diameter
- EFFECT OF CARBON
- EFFECT OF ALLOYS
- Effect of grain size
- HOW HARD IS HARD? (HOW CAN WE TELL HOW HARD EACH
GRADE WILL GET?)
- Use of Jominy or Hardenability Graphs (Hardness vs Distance)
- (Tempering would be added in a two day presentation....)
- HARDNESS VS. HARDENABILITY
- EFFECT OF CARBON: GIVES HARDNESS AT SURFACE, Makes core
hardness possible in small sections, or in larger sections if alloys are present.
- EFFECT OF ALLOYS: GIVES HARDENABILITY IN CORE OF LARGE
PARTS or in material that is cooled at moderate rates.
- EVALUATING HEAT TREATING QUALITY
- HARDNESS PROFILES
- Microhardness (Rockwell C 50 Equivalent criterion)
- EVALUATION BY MICROSCOPE (This is usually extremely rushed in a one
- Specimen Preparation Procedure
- Concept of MICROSTRUCTURE being a witness to thermal history of
- Specimen Preparation Procedure
- Evaluation for UNIFORMITY of carbon distribution
- Evaluation for IMPERFECTIONS
- UNDISSOLVED CARBIDES due to insufficient "Time at
- INTERGRANULAR FERRITE due to "slack quench"
- INTERGRANULAR CARBIDES often due to excessive carbon
in carburizing atmosphere
- RETAINED AUSTENITE due to many things. Some (high
nickel alloys) grades more susceptible than others.
- DECARBURIZATION due to insufficient amounts of carbon in
atmosphere during any exposure to heat, especially red heat.
Can come from processing prior to heat treating.
- Some comments on residual stresses and residual stress measurement
techniques would be in a two day presentation....
- CHOOSING A STEEL AND HEAT TREAT CONDITION FOR A GIVEN
- HIGH CARBON STEEL
- Wear applications
- MEDIUM CARBON STEEL
- Strength critical applications
- LOW CARBON STEEL
- Applications requiring welding, forming or a lot of machining
- Structural steel
- Easiest to process
- PLAIN CARBON STEEL
- Least Expensive
- Most Available
- ALLOY STEEL
- More Expensive
- Best Crack Resistance in Hardened Parts
- HARDEN, NORMALIZE OR ANNEAL
- Parts will often be annealed or normalized for machining and then
hardened for service use.
- A normalized steel with higher carbon may SOMETIMES perform the
same function as a hardened steel of lower carbon
- DISTORTION/CRACKING PROTECTION REQUIREMENTS
- Avoid sharp corners, radii, etc.
- Avoid large differences in section thickness, diameter, etc.
SEE INFORMATION PROVIDED BY GOOD METALS.
- USE SPECIAL HARDENING PROCEDURES
- PREVENTION OF HYDROGEN EMBRITTLEMENT
- Keep hardness to below HRC 40.
- Plating Procedures
- SELECTION OF A HEAT TREATER
- Small or Large Lots
- Small or Large Parts
- Traceability Requirements
- Testing Requirements
- Process Records
- Part Evaluations
- Equipment Considerations
- Experience with particular operation desired
- What not to do when specifying heat treated steel parts or
"What heat treaters won't like if you do"
Back to the Seminar Page
- Specify process and results
- Specify hardness ranges out of line with what the material is designed for
- Specify an invalid hardness test method
- Specify too tight a range for hardness, case depth, etc.
- Use non standard practices without adequate testing
- COUNT ON YOUR HEAT TREATER TO GUESS HOW CRITICAL YOUR
- What not to expect when purchasing heat treating services
- Similar results out of different furnaces
- Inspection when not negotiated
- Results not consistent with available technology level of equipment or
knowledge of personnel