Notes on Treatments

Ac1 Temperature at which austenite starts to form, during heating (see formula).
Ac3 Temperature at which the transformation of ferrite into austenite stops, during heating
(see formula).
Ms Temperature at which the transformation of austenite into martensite starts, during cooling (see formula). usually Ms = 539 – (423xC% ) – (30,4xMn%) – (12,1xCr%) – (17,7xNi%) – (7,5xMo%) = ….. °C + 20 only if all the alloyng elem. are completely dissolved in the austenite Ms = 561 – 474C – 33Mn – 17Ni – 17Cr – 21Mo for high-alloy Ms = 550 – 350C – 40Mn – 20Cr – 10Mo – 17Ni – 8W – 35V – 10Cu + 15Co + 30Al
Mf Temperature at which the transformation of austenite in martensite is complete, martensite finish (see formula). Mf = Ms – 215 °C = ….. °C + 15
Austenitization Operation during which the material is heated to a temperature at which the structure becomes austenitic.
Annealing The main requirement of this heat treatment is to achieve a lowering of the hardness of hot deformed, rolled and cold drawing materials. Annealing is introduced in some cases to eliminate stresses or non-homogeneous structures.
Beta annealing This treatment produces in some titanium alloys the beta phase through heating at an adequate temperature, followed by a controlled cooling in

order to prevent its decomposition.
Bright annealing An annealing process that is carried out in a controlled atmosphere furnace (ex. nitrogen) or vacuum in order that oxidation is reduced to a

minimum and the surface remains relatively bright.
Full annealing Heating at a temparature of 20-50 °C above the critical range Ac3 for hypoeutectoid steels, Ac1 for hypereutectoid steels, pause at this temperature for a suitable time, 2-3 minutes each mm of thickness. A very slow cooling in the temperature field inside the critical range Ac3-Ac1. Final cooling at ambient temperature with speeds more or less quick 5 – 30 °C/h as a function of CCT curves.
Homogenization This is carried out at high temperatures of 1100 – 1200 °C with the aim of reducing, by means of diffusion, the heterogeneity of the chemical
annealing compositions due to segregation phenomenon. Risks: burning and coarsening (grain).
Hydrogen annealing The hydrogen dissolved in the molten steel may give rise to internal cracks, the so-called flakes, into steel after it has cooled from the hot working operation. By subjecting the product to an annealing treatment at 600 – 680 °C for several days, followed by slow cooling, the hydrogen content can be reduced and the danger of flakes formation removed. The heat treatment is executed for carbon or alloy steels with hydrogen (H2) greater to 2 ppm. The steels from tools are less sensitive to flakes where also 4 – 5 ppm they do not give problems.
Intermediate Intermediate annealing. An annealing treatment given to wrought metals following cold work hardening for the purpose of softening prior to further
annealing cold-working.
Isothermal annealing This is carried out at 20-30 °C above Ac3 hypoeutectoid steel or Ac1 hypereutectoid steel. This heat treatment enables regeneration of the structure and complete elimination of any work hardening. Heating 50 °C/hour, holding at maximum temperature for an hour per inch of thickness. The furnaces must be provided with forced air circulation and the cooling end under Ac1 must be fast until the temperatures indicated on the technical schedules. Holding until the complete transformation of austenite (min. 2 h) then unloading into air.
Recrystallization Recrystallization annealing. This is recommended for cold-rolled materials and should be carried out at a temperature of Ac1. During the rolling,
annealing the structure tends to orient itself towards the direction of the main deformation.

The hardness increases and the capacity of the steel to undergo further machining, such as drawing, is reduced.
Spherodising Geometrical change of carbide particles, such as plates of cementite, towards a stable spherical shape.
Soft annealing This is carried out at 30 – 50 °C, below Ac1. This treatment does not modify the structure but adequately softens and eliminates the stresses due to previous processes. Cooling (about 10 °C/h) may be carried out either in a furnace or in air.
Stress-free annealing. This is carried out at a temperature of between 600 °C and 650 °C (preferably 650 °C), with cooling in a furnace down to 250-300 °C before being discharged into air.
Sub-critical spheroidizing (annealing) This heat treatment is not recommended for hypoeutectoid steels C < 0,83% to be chip-formed, but it is the only treatment which may make hypereutectoid steels C 0,83 – 2,06% machinable. It is carried out at a temperature near to Ac1 (-10 °C) but they must be held at the temperature for long periods, 1 hour for every 10 mm of thickness of the bundle of bars or rolled and forged products of the entire load. It is an excellent heat treatment for material to be used for cold forming and extrusion
Brittleness This occurs in some hardened and tempered steels after a period of time held at a temperature of 450-525 °C, or during slow cooling within this range of temperatures. Causes loss of toughness.
Carbonitriding Heat and chemical treatment to obtain a higher carbon and nitrogen content on the surface. Both these elements increase the hardness.
Case-hardening Heat and chemical treatment to obtain a higher carbon content on the surface. Carbon increases hardness and contrasts wear.
Case hardening depth This is the distance between the surface and the layer at which the Vickers hardness, measured under a load of 9.81 N, is HV1 = 550
Chromium plating Electroplating treatment to form a film of extremely hard chromium on ground bars. This treatment provides abrasion and corrosion resistance,

and a lower friction coefficient between moving parts; especially with rubber gaskets.
Chromizing Heat and chemical treatment to obtain a higher content of chromium on the surface.
Continuous patenting When the heating and cooling operation are carried out continuously.
Cooling Process for cooling a product by removing heat: nature and temperature of medium-environment, relative movements, etc.
Cooling capacity Ability of a medium (air, oil, polymer, water, salt bath) to carry out a particular cooling programme.
Core tempering Hardening process carried out so that the depth of hardening is not less than the distance between the core and the surface of the product.
Cyaniding Cyaniding. Carbonitriding carried out in a bath of molten salts containing cyanides.
Decarburizing A reduction of carbon in the surface layer of an iron product. The layer thickness may be defined with reference to a structural state or a level of

hardness, or the carbon content in the unaltered base metal.
Deformation Alteration of the initial shape and dimensions of a product, during heat treatment.
Diffusion Operation to spread the elements previously introduced on the surface toward the core of the base metal.
Endothermic atmosphere Furnace atmosphere with an adjustable carbon potential.
Exothermic atmosphere Furnace atmosphere produced exothermically and controlled so that it does not oxidise the material.
Eutectoid A mixture of two or more constituents which forms on cooling from a solid solution and transforms on heating at a constant minimum temperature.
Gaseous case-hardening Treatment in a gaseous medium-environment.
Grain (coarsening) Phenomenon normally caused by high temperatures, much higher than Ac3
Hardenability Aptitude of the steel towards martensitic and/or bainitic transformation.
Hardening and tempering. Hardening and tempering. Hardening treatment composed of quenching and tempering to obtain the desired combination of mechanical properties and good ductility and toughness. See also Quenching and Tempering
Hardening depth This is the distance between the surface and the layer at which the Vickers hardness, measured under a load of 9.81 N, is equal to 80% of the

maximum hardness requested on the surface.
Heat and chemical treatment Process performed in a suitably selected medium-environment to change the chemical composition of the base material.
Heat cycle Change of temperature over time.
Heating Increasing the temperature of a product with a pre-set thermal gradient.
Heat treatment Series of heat operations in order to change the properties and/or the structure of a ferrous material.
Immersion patenting When the material remains wound in bundles or rolls during the heat treatment. The most common cooling mediums are air, lead bath, salt bath and fluid bed.

Induction hardening A widely used process for the surface hardening of steel. The components are heated by means of an alternating magnetic field to a temperature within or above the transformation range followed by immediate quenching. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined, whilst the hardness of the case can be within the range 37/58 Rc. Carbon and alloy steels with a carbon content in the range 0,40 – 0,45% are most suitable for this process.

Ionic nitriding Bombardment of ions. The luminescent discharge occurs in a gaseous mixture, under less than atmospheric pressure, in which the ferrous product constitutes the cathode.

Isotropy Material in which the physical properties (e.g. mechanical characteristics) are identical in all directions.
Ledeburite Aggregation of crystals of the solid phase of austenite and cementite.
Liquid case-hardening Treatment in salt bath.
Macrostructure The general crystalline structure of a metal and the distribution of impurities seen on a polished or etched surface by either the naked eye or under low magnification of less than x10.

Martempering A heat treatment involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the Ms point. Soaking must be long enough to avoid the formation of bainite. The advantage of martempering is the

Microstructure The structure that is observed when a polished and etched specimen of metal is viewed in an optical microscope at magnifications in range of approximately x25 to x1500.

Nitriding Heat and chemical treatment to obtain higher nitrogen content on the surface. Nitrogen increases hardness and contrasts mechanical wear.
Nitrocarburising Heat and chemical treatment to obtain higher nitrogen and carbon content at the surface. Both these elements form a combination layer.
Normalization This is carried out at a temperature just higher than Ac3 (Ac1 for the hypereutectoid steels C% > 0.80) followed by cooling in calm air. The main aim is to homogenise the structure and to reduce the size of the grain enlarged by previous hot transformation operations. This treatment is not recommended for tool steels and self- hardening steels.
Oxidation Precipitation, to a greater or lesser depth, towards the core of the product, of the oxides formed from the oxygen on the surface.
Patenting Austenitization heat treatment followed by cooling to enable subsequent drawing or rolling.
Physical properties Those properties discussed in physics (exclusive of those described as mechanical properties), for example, density, electrical conductivity, coefficient of thermal expansion. The term physical properties is often used incorrectly to describe mechanical properties

Polymer Polymer. Synthetic fluid consisting of water and organic products with a high molecular weight (polymers). The rate of tempering, with a 35% solution, is just greater than that of oil and it enables a greater range of steels to be treated without risk of failure. These baths are replacing tempering oil, which is more pollutant and costly.
Pre-heating This consists in heating the material to and holding for a certain period of time at one or more intermediate temperatures between the initial temperature and the maximum planned temperature.

PWHT Post-weld heat treatment.
Quenching This is the cooling of a ferrous product faster than in calm air. It is good practice not to use a quenching medium which is more drastic than necessary as the faster the cooling the greater will be the stresses induced in the part. Quenching baths must be stirred to prevent vapour bubble adhering to the material. The most commonly used baths are: gas mixtures (for sub-zero treatment), water, salt baths, polymers (water with that of the material to be quenched. The temperature of the bath at the start max 22 °C, at the end of quenching must not exceed 49 °C.

Residual stress The stress which exists in an elastic solid body in the absence of, or in addition to, the stresses caused by an external load. Such stresses can arise from deformation during cold working such as cold drawing or stamping, in welding from weld metal shrinkage, and in changes in volume due to thermal expansion.

Salt bath A method of heating steel using a bath of molten salts. Salt baths give uniform heating and prevent oxidation, they are used for hardening, tempering or quenching. The type of salt used depends on the temperature range required. For hardening, sodium cyanide, sodium carbonate and sodium chloride are in common use.

Self hardened Cooling by heat conduction towards unheated parts.
Self hardening Steel with hardenability such that cooling in air gives a martensitic structure to large-sized parts.
Secondary hardening Hardening achieved after one or more temperings which precipitate a compound or give rise to the formation of martensite or bainite starting from the residual austenite.

Silicization Heat and chemical treatment in order to increase the silicon content on the surface.
Solid case-hardening Treatment in a solid medium-environment. It is performed in closed metal containers.
Solubilization (Hardening of austenitic steels) This treatment is usually carried out at 1000 – 1100 °C with subsequent rapid cooling in water or forced air.
Stabilizatione A term applied to a number of processes: a) A type of heat treatment to relieve internal stresses: b) The retarding or prevention of a particular reaction by the addition of a stabilising element; c) A thermal and/or mechanical treatment given to magnetic material in order to increase the permanency of its magnetic properties or condition.

Stress relief. Treatment to reduce stresses without reducing the hardness. It is generally carried out at 50 °C below the last tempering carried out on the parts. Bright stress relief is carried out in a controlled atmosphere furnace or vacuum in order that oxidation is reduced to a minimum and the surface remains relatively bright.

Sulphocarbonitriding Heat and chemical treatment in order to increase the sulphur, carbon and nitrogen content on the surface.
Tempering This is the heat treatment which a ferrous product undergoes after hardening by quenching, to achieve the desired mechanical properties. After quenching, the material is highly stressed and these stresses must be eliminated as their force, if it exceeds the failure load, could break the material. This is one of the purposes of tempering. The second is to lower the strength until a compromise is reached between a good failure load and good toughness (impact strength). Useful experience: Considering 0 to be the strength after 2 hours of tempering, the variations in N/mm2 obtained by increasing and reducing the times maintained at the temperature are shown below.

Duration of tempering h 30′ 1 2 3 4 5 7 10
Variation in strength N/mm2 .+60 .+30 0 -30 -45 -50 -60 -80
Tempering diagram Diagram showing the relationship between the mechanical characteristics and the tempering temperature for a specific period of time at that temperature.

Time at temperature Time from when the part has reached the pre-set temperature at the core to the next change in temperature. A period of ½ hour per inch of thickness is normally used during quenching and 1 hour during tempering. These parameters apply to forged parts in general and thicknesses greater than 50 mm.

Other method

Temperature K Temperature K Temperature K Temperature K Temperature K
575 °C 21 650 °C 31 750 °C 43 825 °C 51 900 °C 60
600 °C 25 675 °C 34 775 °C 46 850 °C 54 925 °C 63
625 °C 28 700 °C 37 800 °C 49 875 °C 57 950 °C 66

Maximum thickness mm / K = hours of active time at temperature

Example: to normalise a Ø 400 mm at 900 °C (K = 60) -> 400/60 = 6.6 h

for compact rolls, maximum time increased by 50%

for bundles of compact bars, times increased by 30%
Transformation diagrams Series of time/temperature diagrams which define the initial and final moments of the transformation of austenite under isothermal conditions for each temperature level.

Transition temperature The temperature at which a transition from ductile to brittle fracture takes place in steel. It is usually determined by making a series of Charpy impact tests at various temperatures, the transition temperature is usually taken as the point where 50% of the fracture is brittle

T.T.T. Time Temperature Transformation curve. An isothermal transformation diagram showing the relationship between temperature and the time taken for the decomposition of austenite when the transformation occurs at constant temperature.

Wear-resistant coating Deposit of thin layers of very hard compounds which resist wear and corrosion, such as.

CVD Chemical Vapour Deposition

DLC Diamond Like Carbon.

HIP Hot Isostatic Pressing (application of heat and pressure to a metal)

PCD Poli Crystalline Diamond (graphite, under pressure and at high temperature, mixd with Co or Ni).

PVD Physical Vapour Deposition