Heat Treatment of Steel

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Heat Treatment of Steel

  • Most heat treating operations begin with heating the alloy into the austenitic phase field to dissolve the carbide in the iron.
  • Steel heat treating practice rarely involves the use of temperatures above 1040 C
  • Classification
  • Heating and rapid cooling(quenching)
  • Heating and slow cooling

Purpose of heat treatment

  • Improvement in ductility
  • Relieving internal stresses
  • Grain size refinement
  • Increase of strength and hardness
  • Improvement in machinability and toughness

Factors involved

  • Temperature up to which material is heated
  • Length of time that the material is held at the elevated temperature
  • Rate of cooling
  • The surrounding atmosphere under the thermal treatment

Effects of Heat Treatment

Iron Carbide Phase Diagram

Types of Heat Treatment

Annealing Process

  • Material is exposed to an elevated temperature for an extended time period and then slowly cooled, allowing phase changes.
  • Utilized for low-carbon and medium-carbon steels.
  • Full Annealing
  • Process Annealing or Stress Relief Annealing
  • Spheroidising

Purposes of Annealing

Relieve Internal Stresses

  • Internal stresses can build up in metal as a result of processing. Such as welding, cold working, casting, forging, or machining.
  • If internal stresses are allowed to remain in a metal, the part may eventually distort or crack.
  • Annealing helps relieve internal stresses and reduce the chances for distortion and cracking.

Process Annealing (Intermediate Annealing)

  • A heat treatment used to negate the effects of cold work, i.e., to soften and increase the ductility of a previously strain-hardened metal.
  • In process annealing, parts are not as completely softened as they are in full annealing, but the time required is considerably lessened.
  • Process annealing or stress-relief annealing is frequently used as an intermediate heat-treating step during the manufacture of a part.
  • Recovery and recrystallization processes occur during the process

Stress relief Annealing

  • Internal residual stresses may develop in metal pieces due to
    • Plastic deformation processes(machining and grinding)
    • Non-uniform cooling of a piece that was processed or fabricated at an elevated temperature(welding or casting)
  • Distortion and warpage may result if these residual stresses are not removed.
  • The material is heated to the recommended temperature, held long enough to attain a uniform temperature, and finally cooled to room temperature slowly.
  • The annealing temperature is ordinarily a relatively low one such that effects resulting from cold work and other heat treatments are not affected.

Normalising

  • Heating the alloy to 55 to 85 degree Celsius above the A3 or Acm and holding for sufficient time so that the alloy completely transforms to austenite, followed by air cooling
  • To refine the grains and produce a more uniform and desirable size distribution for steels that have been plastically deformed
  • Normalising does not soften the material as much as full annealing does
  • The cooling process does not leave the material as ductile or as internally stress-free
  • A normalized part will usually be a little stronger, harder, and more brittle than a full-annealed part.

Hardening

  • Hardening of steels done to increase the strength and wear resistance
  • Heated to 30-50 C above the upper critical temperature and then quenched
  • The quicker the steel is cooled , the harder it would be

The steels shown in blue can be heat treated to harden them by quenching.

Hardening Temperatures

  • The temperatures for hardening depend on the carbon content.
  • Plain carbon steels below 0.4% will not harden by heat treatment.
  • The temperature decreases from approx 820 C to 780 C as carbon content increases from 0.4% up to 0.8 %.
  • Above 0.8 % the temperature remains constant at 780 C
  • Hardening temperature same as that for normalizing

Quenching Media

Four commonly used quenching media:

  • Brine – the fastest cooling rate
  • Water -moderate cooling rate
  • Oil -slowest cooling rate
  • Gas – used in automatic furnaces, usually liquid nitrogen can be very fast cooling.

Too rapid cooling can cause cracking in complex and heavy sections

Hardenability

  • The hardenability of steel is broadly defined as the property which determines the depth and distribution of hardness induced by quenching.
  • This is dependent upon the chemical composition of the steel alloy.
  • The addition nickel, chromium and molybdenum will slow the transformation to other phases and allow more martensite to form.
  • Most heat treatable steels are alloys rather than plain carbon steels.

Depth of Hardening

  • Due to the mass effect, not all the section of a large component may be hardened due to too slow of cooling rate.
  • This may leave a soft core, or in extreme cases prevent hardening altogether.

TTT Diagram

Some related terms

  • Retained Austenite – Austenite that is unable to transform into martensite during quenching because of the volume expansion associated with the reaction.
  • Tempered Martensite
  • Marquenching

Quench Cracks

Tempering

  • The brittleness of martensite makes hardened steels unsuitable for most applications.
  • Different cooling rates between edge and core of components result in internal stresses.
  • This requires the steel to be tempered by re-heating to a lower temperature to reduce the hardness and improve the toughness.
  • This treatment converts some of the martensite to bainite.

Tempering Temperatures

Isothermal Heat Treatments

  • Austempering -The isothermal heat treatment by which austenite transforms to bainite.
  • Isothermal annealing – Heat treatment of a steel by austenitizing, cooling rapidly to a temperature between the A1 ant the nose of the TTT curve, and holding until the austenite transforms to pearlite.

Austenite

  • This is the structure of Irons and Steels at high temperature(Over 800 deg Celsius)
  • For Quench Hardening all the material must start as Austenite
  • Quenching cause the Austenite to be partially or totally transformed to martensite

Martensite

  • Only formed by very rapid cooling from the austenitic structure.
  • Needs to be above the cooling rate

The needle-like structure of martensite, the white areas are retained austenite.

Introduction To Tool Steel

  • TOOL STEEL are high quality steels made to controlled chemical composition and processed to develop properties useful for working and shaping of other materials
  • The carbon content in them is between 0.1 – 1.6%. Tool steel also contains alloying elements like, chromium, molybdenum and vanadium.
  • Tool steel offers better durability, strength, corrosion resistance and temperature stability, as compared to the construction & Engg. Steel.
  • These are used in applications such as Blanking, die forging, Forming, extrusion and plastic molding etc.,

Types

Hot Worked Tool Steels

  • Carbon-content =0.3-0.5%.These Steels are used for high temperature metal forming operation (except cutting), where the temperature is around 200-800 degree Celsius.
  • These are characterized by high hot yield strength, high red hardness, wear resistance, toughness, erosion resistance, resistance to softening at elevated temperatures, good thermal conductivity.
  • These are divided into 3 groups depending on the principle alloying elements:
    • Chromium-based [H11-H19]
    • Tungsten-based [H20-H26]
    • Molybdenum-based [H41-H43]

Chromium-Based

  • Contains Chromium (>=3.25%), and small amounts of vanadium, Tungsten, and molybdenum.
  • These are characterized by High red Hardness & high hardenability.
  • Oil quenching is required when dimensional stability is not of prime importance. Tempering temperature for these steels varies from 550-675 degree Celsius.
  • Applications:
    • Hot dies for extrusion, forging, mandrels, punches.
    • Highly stressed structural parts of supersonic aircrafts.
    • Hot work steels

Tungsten Based

  • Contains Tungsten (=9.00%) & Chromium (2.0 -12.0%), and low carbon %.
  • These are characterized by resistance to high temperature softening.
  • Tempering temperature for these steels varies from 550-675 degree Celsius.
  • Applications:
    • 1. Punches
    • 2. mandrels
    • 3. Extrusion dies for Brass, Steel &Nickel alloys.

Molybdenum Based

  • Contains Molybdenum (8.00%) & Chromium (4.0-12.0%), and some tungsten and vanadium.
  • These are characterized by high toughness & high heat check resistance.
  • Tempering temperature for these steels varies from 550-650 degree Celsius.

Tungsten based

  • Contains Tungsten (=9.00%) & Chromium (2.0 -12.0%), and low carbon %.
  • These are characterized by resistance to high temperature softening.
  • Tempering temperature for these steels varies from 550-675 degree Celsius.
  • Applications:
    • 1. Punches
    • 2. mandrels
    • 3. Extrusion dies for Brass, Steel &Nickel alloys.

Molybdenum based

  • Contains Molybdenum (8.00%) & Chromium (4.0-12.0%), and some tungsten and vanadium.
  • These are characterized by high toughness & high heat check resistance.
  • Tempering temperature for these steels varies from 550-650 degree Celsius.

Vacuum-Hardening of Hot-Work Tool Steel

Microstructure of H-13 Tool Steel (1000x)

Microstructure of Annealed H-13 Tool Steel

NADCA Specification #207-2006

Banding/Microsegregation Chart

Annealed Quality Microstructure chart

Heat Treatment Quality Microstructure Chart

Cold Worked Tool Steels

  • These steels are used for making tools for cold work applications, when the tool surface temperature does not rise more than 200 degree Celsius.
  • These are characterized by high abrasion & wear resistance, higher toughness and high impact resistance.
  • These steels are also called “Non-distorting steels”, as they show little change in dimension during heat treatment.
  • These are divided into 3 groups:
    • Oil hardening Steels [GRADE ‘O’]
    • Air hardening Steels [GRADE ‘A’]
    • High Carbon, High Chromium Steels [GRADE ‘D’]

Oil-hardening Steels

  • These are hardened by oil-quenching & contain high carbon with manganese, chromium, molybdenum.
  • These are characterized by high machinability, wear resistance, and non-distorting properties.
  • Tempering temperature for these steels varies from 100-425 degree Celsius.
  • Applications:
    • Taps
    • Blanking and forging dies
    • Threading dies
    • Expansion reamers

Air-hardening steels

  • These are hardened by air-quenching & contain carbon (1.0%) with manganese, chromium, molybdenum & tungsten.
  • These are characterized by high wear resistance & high hardenability, fair red hardness, good toughness and resistance to decarburization.
  • Tempering temperature for these steels varies from 150-425 degree Celsius.
  • Applications:
    • Knives
    • Blanking and Trimming dies
    • coining dies

High Carbon, High Chromium Steels

  • These are hardened by oil- or air-hardening & contain carbon (1.4-2.3%) & chromium (12-14%), with molybdenum, cobalt, vanadium.
  • Vanadium prevents these steels form showing grain coarsening (up to 1040 degree Celsius). Chromium imparts non-deforming properties. Tempering of these steel results in high hardness, wear & abrasion resistance.
  • Tempering temperature for these steels varies from 150-375 degree Celsius.
  • Applications:
    • Mandrel for tube rolling by pilger rolls.
    • Blanking and piercing dies, coining dies, drawing dies

High Speed Tool Steels

  • These are highly alloyed tool steels developed initially to do high speed metal cutting. Now they used in a wide variety of machining operations.
  • These are characterized by high hardness (60-65 HRC at 600-650 degree Celsius), high red hardness, wear resistance, reasonable toughness and good hardenability.
  • They contain 0.6% carbon, 4% chromium, 5-12% cobalt.
  • Carbon imparts hardness of at-least 60 HRC of martensite formed. Chromium increase hardenability & corrosion resistance. Cobalt increases the thermal conductivity, melting point, red hardness & wear resistance of high speed steels.
  • These are divided into 2 groups depending upon the principal alloying elements & the composition:
    • Molybdenum high speed steel [GRADE ‘M’]
      (Contain molybdenum, tungsten, chromium, vanadium & sometimes cobalt)
    • Tungsten high speed steel [GRADE ‘T’]
      (Contain high amount of tungsten with chromium, vanadium and some cobalt)

Application

  • End mills, drills, lathe tools, planar tools.
  • Punches, reamers
  • Routers, taps, saws
  • Broaches, chasers, and hobs.

Typical heat treatment cycle for HSS