Technical Heat Treatments: Overview

Technical Heat Treatments

• According to IVW and DIN EN 10052, heat treatment is a process where a workpiece is exposed to temperature-time cycles and physical or chemical influences to achieve desired characteristics.
• Heat treatment processes are divided into thermal, thermo-chemical, and thermo-mechanical categories.
• Annealing influences microstructure to improve processing properties like machinability.
• Hardening and tempering optimize end-usage properties like hardness and wear resistance.
• Change in size, shape, and orientation of microstructural components can be achieved and internal stress can be reduced
• Microstructural components can be transformed to achieve equilibrium (normalizing) or non-equilibrium (hardening).
• Every heat treatment includes heating to a target temperature, maintaining it (holding), and cooling.
• Heat treatment is represented via temperature-time diagrams.
• Material properties, workpiece size/shape, and process duration impact heat treatment selection.
• Increased heating rate and component size cause larger temperature differences between the surface and center.
• Poor thermal conductivity amplifies temperature differences and thermal stresses.
• The heating rate must suit the material and dimensions to avoid distortions and stress cracking.
• The cooling rate must be selected to achieve the desired microstructure with low residual stresses.
• Holding time is usually based on experience.
• For non-alloyed steels, temperature ranges rely upon carbon content in the iron-carbon diagram.
• In steels with more alloying elements, transformation temperatures shift

Hardening

• The goal is to produce a martensitic microstructure, characterized by high hardness
• Hardening also applies when a small amount of bainite appears alongside martensite.
• Complete transformation to martensite may not always be possible

Definitions of Hardening

• Hardening: Austenite transforms completely or partially into martensite/bainite; requires steel with ≥ 0.3% carbon

• Case Hardening: Carburizing/carbonitriding followed by hardening; employs low carbon content steels(<0.25%)

• Hardenability: Steel's capacity to increase hardness through martensite transformation across a cross-section.

• This references maximum achievable hardness under ideal conditions and hardness distribution.

• Hardness penetration: The depth from component surface to a specified microstructure (e.g. martensite) or predefined hardness

• Construction materials and tools often undergo additional tempering after hardening

• Tempering optimizes strength for stress/strain conditions or minimizes cracking risk.

• High-temperature tempering balances strength and toughness.

• After quenching/tempering, steel properties are determined by: austenitization, quenching, tempering, chemical composition, and component dimensions.

Quenching and tempering

• A combined heat treatment with these steps:

• Austenitization: The heated component is penetrated and its microstructure is homogenized.

• Quenching: Rapid cooling occurs by using oil, water or air medium.

• Tempering: Re-heating takes place and sits at a temperature below Ac1.

• There is a relationship between carbon content and the maximum possible hardness when microstructure consists almost entirely of martensite.

• Maximum hardness = K + 50 * (mass-% C) ± 2 in HRC

• Achievable hardness in the center of a workpiece depends on the cooling rate, which decreases from surface to core.

• Cooling rate differences lead to varied microstructures based on steel's TTT diagram.

• An indication for high hardening is depth is when the austenite transformation to pearlite/bainite is suppressed through high cooling rate.

• The desired core hardness is easier to achieve that way.

• In contrast, there is a low hardness penetration if the microstructure does not fully transform to martensite, even with abrupt cooling.

• Hardening capacity depends on carbon content.

• Hardness penetration depends on alloying elements.

• Elements slowing pearlitic transformation in steels increase hardness penetration like molybdenum, chromium, and manganese.

• Carbon increases hardness penetration slightly.

• Austenite grain size affects hardening.

• Larger grains increase the hardness penetration by reducing the number of transformation nuclei and increasing transformation inertia.

Austenitization

• Hardening capacity depends on carbon content dissolved in austenite before quenching
• You must consider the following rules to choose the best possible temperature
• Non-alloyed hypoeutectoid steels: Hardening temperature should be 30-50°C above the GS-line in the Fe-C diagram.
• That guarantees complete dissolution of soft ferritic microstructure components and even distribution of carbon.
• Non-alloyed hypereutectoid steels: Austenitize above the SK-line at 780-800°C.
• For these steels >0.8%C, the Mr-temperature is below room temperature.
• After quenching in the water, a fully martensitic material cannot be produced from homogenous austenite - retained austenite remains in the material, reducing hardness.
• After quenching from the heterogenous + Fe3C area, the hardened material consists of fine, needle-like martensite with embedded carbides and small amount of retained austenite.

Quenching

• This can be carried out by utilizing various mediums

• The cooling rate in all parts must be above the upper critical rate, in order to guarantee a fully martensitic material

• It is critical to suppress the formation of pearlite and bainite, by what's mainly influenced by these:

  • Hardening temperature and holding time: An increase in temperature decreases the critical cooling rate
  • Steel composition: Critical cooling rate lessens as the carbon content (up to 0.9%) increases

• A reduction of the critical cooling rate shifts to the TTT diagram to the right. At carbon content higher than 0.9% C the increase in critical cooling is cause by carbides.

• The undissolved carbides serve as nucleation spots for pearlite transformation, which speed up the process.

• The cooling rates in the zones depend on:

  • The specific heat capacity and thermal conductivity of steel
  • The size, shape and quality of the component
  • The heat transfer in the workpiece

• Decreasing quenching effects occur with these mediums: salt-water bath, water, polymer solution, oil, hot bath and air.

• Desired quenching effects are achieved by dissolving salts in water or by using acids, bases, or types of oils and emulsions, and hot baths

• Polymer solutions are increasingly becoming the quenching medium choice. They replace the oils and water.

  • Advantages:
  • No fire danger
  • No smoke or steam development
  • No surface damage from the oil
  • Enhanced intensity, combined with cheaper, low-alloyed steel.
  • No need to degrease the hardened pieces

• Reduced intensity results in fewer distortions and crack formation

• Air or a hot-bath as a medium results in heat dissipating through conduction and convection

• Those mediums have the greatest cooling effect just after cooling begins, and it reduces with temperature

• Cooling takes place in three stages when using water, solutions and oils; film, boiling and convection phase

• First a film forms around the component, which acts as an insulator

• Then the steam film breaks, with intense bubbling, causing increase in the rate

• Heat flow only occurs through convection

• Adding salt increase rate and suppresses the steam film. Boiling process is more uniform.

• Transformation is completed within the boiling phase, to prevent transformations in the core.

• Maximizing the heat has the greatest effect during boiling. This is done by burst steam bubbles that hinder heat flow.

Austempering

• Produces a state in which ferrite and pearlite are avoided, and partial or complete transformation into bainite occurs.
• Bainite is harder and stronger than pearlite and ferrite, but might not attain the hardness as hardening
• Requires the component to be heated and kept at the austenitizing temperature for austenite
• The hardening lower limit is usually used as the austenitizing temperature
• You must use steels with a hardenability to use effects from this
• For this, use steel alloyed with molybdenum, chromium, manganese, and Boron
• This transformation does not turn into ferrite or pearlite during cooling
• Can be obtained through cooling at sufficient speed for transformation into bainite
• OR, through isothermal transformation
• For transformation, rapid cooling from the austenitizing temperature to just under 500°C is needed
• This must be held until the transformation is complete, then cooled at any rate.

Tempering

• This consists of the heating a hardened component up to a temp below A1, which lead to a more stable martial
• This improves values but strength is reduced
• Tensile and yield reduce with temperature, while the area and notch impact increase 1st tempering level: at 100-150°C, ɛ-carbides precipitate out of martensite if carbon content is over 0.2%. 2nd tempering level: at 250 to 325°C, retained austenite transforms to bainite or martensite. 3rd tempering level: at 325 to 400°C, cementite forms and e-carbides transform to Fe3C. 4th tempering level: above 400°C, recovery and recrystallisation happens 5th tempering level: above 450°C, alloy carbides form.
• These ranges can overlap and depend on the elements and heating rate. If at a long time the microstructure is a uniformed carbide
• Depending on the composition, toughness can be reduced in temperature
• Tempering brittleness, also known as 300°C, is associated with carbides and segregation by P, Sb, As and Sn
• Increased through segregation to the grain, strengthens by the rejection from carbides

Corrective measures for brittleness:

1. Annealing below Acı, where they can't re-form again. But it only applies on resistant steels
2. Raising grains that increases the area degree which reduces locals
3. Reducing waste
4. Alloying that stops segregation
5. Bonding the phosphorus

• Resistance is the ability to maintain qualities over temperatures and it is decided by the alloying element content
• With levels, a hardness and reduction occurs
• Hot forming and steels are extreme examples; reaching a maximum between 500 and 600°C, even over the quenching harness
• Reduction in the face is the greatest and the ones that are closest decrease
• Tempering of martensite increases to the formation temperature

Examination of hardenability

• According to DIN EN ISO 18265, this is measured using the end-quench test (Jominy).

1. Austenitisation of the sample;
2. Quenching from the face-side with water;

• Heat flows in direction and cooling rate decreases with distance leading to reduced hardness values. The values change with the long axis
• The hardness of the quenched face (100% martensite) is a measure for the hardening capacity.
• While carbon content is relevant, hardness penetration depends on alloying elements.
• Influence of austenitising: high temperature raises hardening capacity and raises penetration.
• Used when suitable for specific quality standards

Calculating

• Hardenability is mainly a function of the chemical composition
• Equation 5.4: Jx = bo + bc * %C+ bsi * %Si + bMn * %Mn +

Case hardening

• This is applied to elements, of which require a separation (toughness) and the surface layer (wear) and the economic efficiency (Figure 5.20)
• Series of processes that range from mechanical, thermal, thermochemical
• This relies upon the stress profile, large material and process target value
• It is a diffusion that is widely used
• This is utilized to increase stress, and fatigue life
• To increase the strength, the content will range from 0.1 -
• Steel that meet require this include steel
• They are not suited for because of the low hardness. The hardness increase linearly with the content along no residual
• To guarantee machinability, the carbon is increased in the surface layer, carbonization.
• They are summarized (Carburization)
• Compressive stress ensures components head to a reduce stress level (because of the superposition of components)
• Tensile stress lead to increase hardness; high rolling strengths
• Should be carried out at temperatures > 900°C because the iron form has greatest solubility
• Success depends on the rate flow: • Supplying the surface of the material with a sufficient amount of a carbon-donating medium. • Decomposition and absorption at the surface. • Diffusion within the layers to the core,
• The ionized pieces will get their properties after the carbonization
• The way they perform process, depends on the carbon concentration
• Compared, and the higher the start temperatures is different
• The direct is the economical but and growth is prohibited
• Goes through cooling to achieve surface
• Will undergo temperatures then quenched - The austenite trans - For hardness the will be cooled

Properties of hardened materials

• Goal is to transform much to martensite as can
• Level of is determined by the amount and carbon that dissolved
• Maximum harness of steel is 65 HRC. When they are are bainite than values expect, In cast irons the consistent with
• For In sintered the pores hardness is also a can been
• Values use are in the formula for and is the if the between and and are from to The yield also the is

Combined Annealing Processes

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Wire patenting

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Types of Annealing

Annealing

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Coarse

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Spheroidising

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