Basic principles of heat treatment

Heat treatment of a metal or alloy is a technological procedure, including controlled heating and cooling operations, conducted for the purpose of changing the alloy microstructure and resulting in achieving required properties.

There are two general objectives of heat treatment: hardening and annealing.

Hardening

Hardening is a process of increasing the metal hardness, strength, toughness, fatigue resistance.

Strain hardening (work hardening) – strengthening by cold-work (cold plastic deformation).

Cold plastic deformation causes increase of concentration of dislocations, which mutually entangle one another, making further dislocation motion difficult and therefore resisting the deformation or increasing the metal strength.

Grain size strengthening (hardening) – strengthening by grain refining.

Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.

Solid solution hardening – strengthening by dissolving an alloying element.

Atoms of solute element distort the crystal lattice, resisting the dislocations motion. Interstitial elements are more effective in solid solution hardening, than substitution elements.

Dispersion strengthening – strengthening by addition of second phase into metal matrix.

The second phase boundaries resist the dislocations motions, increasing the material strength. The strengthening effect may be significant if fine hard particles are added to a soft ductile matrix (composite materials).

Hardening as a result of Spinodal decomposition. Spinodal structure is characterized by strains on the coherent boundaries between the spinodal phases causing hardening of the alloy.

Precipitation hardening (age hardening) – strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution.

The second phase boundaries resist the dislocations motions, increasing the material strength.

The age hardening mechanism in Al-Cu alloys may be illustrated by the phase diagram of Al-Cu system (see figure below)

When an alloy Al-3%Cu is heated up to the temperature TM, all CuAl2 particles are dissolved and the alloy exists in form of single phase solid solution (α-phase). This operation is called solution treatment.

Slow cooling of the alloy will cause formation of relatively coarse particles of CuAl2 intermetallic phase, starting from the temperature TN.

However if the the cooling rate is high (quenching), solid solution will retain even at room temperature TF. Solid solution in this non-equilibrium state is called supersaturated solid solution.

Obtaining of supersaturated solid solution is possible when cooling is considerably faster, than diffusion processes.

As the diffusion coefficient is strongly dependent on the temperature, the precipitation of CuAl2 from supersaturated solution is much faster at elevated temperatures (lower than TN).This process is called artificial aging. It takes usually a time from several hours to one day.

When the aging is conducted at the room temperature, it is called natural aging. Natural aging takes several days or more.

Precipitation from supersaturated solid solution occurred in several steps:

Segregation of Cu atoms into plane clusters. These clusters are called Guinier-Preston1 zones (G-P1 zones).

Diffusion of Cu atoms to the G-P1 zones and formation larger clusters, called GP2 zones or θ” phase. This phase is coherent with the matrix .

Formation of θ’ phase which is partially coherent with the matrix. This phase provides maximum hardening.

Annealing

Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature (annealing temperature), followed by controlled cooling.

Annealing results in relief of internal stresses, softening, chemical homogenizing and transformation of the grain structure into more stable state.

Annealing stages:

Stress relief (recovery) – a relatively low temperature process of reducing internal mechanical stresses, caused by cold-work, casting or welding.

During this process atoms move to more stable positions in the crystal lattice. Vacancies and interstitial defects are eliminated and some dislocations are annihilated.

Recovery heat treatment is used mainly for preventing stress-corrosion cracking and decreasing distortions, caused by internal stresses.

Recrystallization – alteration of the grain structure of the metal.

If the alloy reaches a particular temperature (recrystallization or annealing temperature) new grains start to grow from the nuclei formed in the cold worked metal. The new grains absorb imperfections and distortions caused by cold deformation. The grains are equi-axed and independent to the old grain structure.

As a result of recrystallization mechanical properties (strength, ductility) of the alloy return to the pre-cold-work level.

The annealing temperature and the new grains size are dependent on the degree of cold-work which has been conducted. The more the cold-work degree, the lower the annealing temperature and the fine recrystallization grain structure. Low degrees of cold-work (less than 5%) may cause formation of large grains.

Usually the annealing temperature of metals is between one-third to one-half of the freezing point measured in Kelvin (absolute) temperature scale.

Grain growth (over-annealing, secondary recrystallization) – growth of the new grains at the expense of their neighbors, occurring at temperature, above the recrystallization temperature.

This process results in coarsening grain structure and is undesirable.

Annealing

Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature (annealing temperature), followed by controlled cooling.

Annealing results in relief of internal stresses, softening, chemical homogenizing and transformation of the grain structure into more stable state.

Annealing increases an extent of equilibrium of the metal structure resulting in softening and high ductility.

Annealing temperature and the control cooling rate depend on the alloy composition and the type of the annealing treatment.

The following types of annealing are used in heat treatment of alloys:

Full annealing is a process in which a ferrous alloy (commonly hypoeutectoid steel) is heated to about 100°F (55°C) above the upper critical temperature, followed by soaking and slow cooling in the furnace or in some medium to a temperature below the critical temperature.

For the non-ferrous alloys full annealing means full softening after cold work in contrast to partial annealing meaning partial softening.

Subcritical annealing is annealing of cold-worked steel below the critical temperature on the iron-carbon phase diagram.

Recrystallization annealing is a process of heating a cold worked metal to a temperature above the recrystallization temperature followed by soaking for a time required for the grain structure transformation.

Recrystallization annealing is widely used as an intermediate softening treatment between stages of cold work (cold rolling, drawing).

Combination of recrystallization annealing and cold work allows to control the microstructure grains size.

Stress relief (recovery) – a relatively low temperature process of reducing internal mechanical stresses, caused by cold work, casting or welding.

The stress relief temperature is lower than the recrystallization temperature.

Spheroidizing annealing is a process of controlled heating and cooling high carbon steels (tool steels) to produce spherical (globular) form of cementite inclusions.

This treatment improves the machining characteristics of the steel.

Bright annealing is an annealing treatment which is carried out in furnaces with reducing atmosphere preventing surface oxidation of the steel parts.

Homogenizing annealing is a durable high temperature annealing treatment intended to decrease chemical segregation by diffusion.

Homogenizing annealing is used for steel and aluminum ingots and castings.

More homogeneous intercrystalline distribution of carbon, phosphorus sulfur and alloying elements in steel ingots is achieved in annealing at 2000°F -2370°F (1100°C - 1300°C) for 20-50 hrs.

Aluminum alloys are treated at 790°F - 970°F (420°C - 520°C) for 16-30 hrs.

NormalizingNormalizing is a process in which a steel is heated to about 100°F (55°C) above the upper critical temperature, followed by soaking and cooling in still air at room temperature.

Normalizing treatment is similar to the full annealing treatment. The difference is in the cooling method and rate – full annealing involves slow controlled cooling if the furnace or in some medium providing slow cooling rate.

As normalizing requires less time, it is more economically efficient heat treatment method than full annealing.

Normalizing relieves internal stresses caused by cold work while grain growth is limited by the relatively high cooling rate therefore the mechanical properties (strength, hardness) of a normalized steel are better than in an annealed steel.

Since the cooling rate in the normalizing heat treatment is not controlled, the resulting structure is dependent on the thickness of the steel part, therefore the effect of increased mechanical properties is greater in thin parts.

Quality of surface after machining of a normalized part is also better than in an annealed part. This effect is caused by increased ductility of annealed steel favoring formation of tearing on the machined surface.

Hardening

Hardening is a heat treatment process involving heating a steel above the phase transformation temperature (upper critical temperature, A3), followed by soaking and then rapid cooling (quenching).

When steel is heated above the upper critical temperature, its structure becomes entirely austenitic.

Then the article is cooled at a rate exceeding the critical rate value.

Critical cooling rate is a function of the chemical composition and the grain size of austenite.

If the critical cooling rate is not achieved, a mixture of ferrite and cementite forms.

Depending on the cooling rate the following ferrite-cementite structures may form:

pertlite – ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling in annealing treatment;

sorbite- ferrite-cementite perlite-like structure with finer (than in perlite) grain structure, forming as a result of decomposition of austenite at relatively high cooling rate (cooling in air);

trostite–fine ferrite-cementite perlite-like structure forming as a result of decomposition of austenite at high (but lower than critical) cooling rate (cooling in oil);

bainite– very fine ferrite-cementite mixture, forming in a mechanism similar to the mechanism of martensite transformation, as a result of decomposition of austenite at high (but lower than critical) cooling rate (cooling in a quenching medium);

Cooling in water usually provides cooling rate higher than the critical value.

The structure forming as a result of quenching in water is called martensite (supersaturated solid solution of carbon in α-iron). Martensite is hard and brittle phase, having hardness varying between 500 HB to 710 HB depending on the carbon content.

The temperature interval at which the austenite-martensite transformation occurs is about 480°F - 400°F (250°C - 200°C).

Hardening temperature is the temperature to which a steel is heated before quenching.

If the hardening temperature of a hypoeutectoid steel is at least 100°F (55°C) above the upper critical temperature, quencing will result in complete austenite-martensite transformation (full hardening).

If the hypoeutectoid steel is heated to a temperature, lying between the upper critical temperature (A3) and the lower critical temperature (A1), quenching will result in formation of martensite with some amount of ferrite (partial hardening). This structure is softer than full-hard martensite structure.

In the case of hypereutectoid steel partial hardening results in formation of a mixture of martensite and cementite, which is harder than full-hard martensite structure.

Hardenability is the property of steel indicating the depth to which hardening effect penetrates. Hardenability depends on the chemical composition of the steel, hardening temperature, dimensions and shape of the article and other factors.

Hardenability is detrmined by the Jominy test, in which a steel bar of 1 inch in diameter and 4 inch long is heated above the upper critical point and then one end of the bar is quenched by water jet. Results of the hardness measurements conducted along the bar after quenching indicate the hardenability of the steel.

Isothermal hardening is a hardening method involving quenching in a medium (oil or molten salt) to minimize the part cracking and distortion.

There are two pricipal isothermal methods:

Martempering is the isothermal hardening method, in which a part is quenched in a quenching medium (oil or molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium before the bainite formation.

Austempering is the isothermal hardening method, in which a part is quenched in a quenching medium (oil or molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium after the complete bainite formation.

Case hardening

Case hardening is the diffusion heat treatment operation which involves two stages:

Heating a steel part to a temperature above the upper critical temperature in a medium, containing an element capable to saturate the surface layer of the part through diffusion;

Heat treatment of the part in order to obtain the desired combination of mechanical properties of the hard outer “case” and the ductile “core”.

As a medium for the case hardening solid, liquid and gaseous substances may be used.

The most widely used case hardening methods are: carburizing, nitriding and carbonitriding.

Carburizing Nitriding

Carbonitriding

Carburizing

Carburizing is the process of diffusion enrichment of the surface layer of a part with carbon followed by heat treatment of the part.

As carburizing medium the following substances are used:

Charcoal or other carbon-containing solids mixed with sodium carbonate and barium carbonate accelerating the process of dissolving the carbon in steel.

The process is carried out in steel or cast iron boxes placed into a furnace at the temperature 1650°F - 1750°F (900°C - 950°C) resulting in formation of hard case of the thickness 0.02”-0.08” (0.5mm – 2mm) and containing 0.8-1% of carbon.

Kerosene or benzene – liquid carbonizing mediums, which are usually used in dispersed form;

Methane (CH4), propane (C3H8) – gaseous carbonizing mediums.

The process is carried out in a furnace (batch or continuous) at the temperature 1650°F - 1750°F (900°C - 950°C) for 3-4 hrs.

Thickness of the hard layer formed in the gaseous carburizing may reach 0.15” (4mm).

Heat treatment after carburizing involves hardening-tempering treatments with purpose of controlling structure and properties of both the hard layer and the ductile core.

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Nitriding

Nitriding is the process of diffusion enrichment of the surface layer of a part with Nitrogen.

Gas nitriding is carried out at 930°F - 1110°F (500°C - 600°C) for 40-100 hrs.in the atmosphere of Ammonia, which dissociates to Hydrogen and nitrogen. The latter diffuses into the steel forming nitrides of iron, aluminum, chromium and vanadium.

Ion nitriding (plasma nitriding) is a surface Hardening heat treatment, in which Nitrogen is delivered to the workpiece surface in form of ionized gas (plasma).

The case formed as a result of nitriding has a hardness of about 1100 HV which is higher than the hardness after carburizing.

Nitrided part possess also better wear resistance, increased fatigue strength, enhanced toughness and good resistance to corrosion.

No additional heat treatment is required after nitriding.

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Carbonitriding

Carbonitriding is the process of diffusion enrichment of the surface layer of a part with carbon and nitrogen.

Gaseous carbonitriding is carried out in gaseous medium, consisting of carburizing gas (methane, propane) with addition of 3-8% of Ammonia.

There are two principal methods of the gaseous carbonitriding:

Low temperature gaseous carbonitriding, conducted at 930°F - 1110°F (500°C - 600°C). The steel surface is enriched mostly with nitrogen in this process.

High temperature gaseous carbonitriding, conducted at1470°F - 1750°F (800°C - 950°C). The steel surface is enriched mostly with carbon in this process. This process is followed by heat treatment.

Cyaniding is the carbonitriding process, conducted in molten salt, containing 20-25% of sodium cyanide (extremely toxic substance).

The process is carried out at the temperatures 1500°F - 1580°F (820°C - 860°C) for 1 hour.

Carbonitrided parts possess better (than carburized parts) wear resistance.

Ion nitriding

Ion nitriding (plasma nitriding) is a surface Hardening heat treatment, in which Nitrogen is delivered to the workpiece surface in form of ionized gas (plasma).

Ion nitriding produces high surface hardness, good wear resistance, increased fatigue strength and toughness.

Ion nitriding process Advantages of ion nitriding

Applications of ion nitriding

Ion nitriding process

Ion nitriding is performed in a vacuum chamber at a pressure 1-10 torr.

A DC voltage 100-700 V is applied between the workpiece and the chamber wall. The workpiece is connected to the negative terminal (cathode). The chamber wall is the anode (positive terminal). The wall is usually grounded.

The air is first evacuated from the chamber to 0.1 torr, which is then backfilled with a mixture of nitrogen, a Hydrogen containing gas (eg., methane) and an inert gas. The gas mixture is continuously supplied to the chamber, pressure of which (1-10 torr) is controlled by the gas flow rate and the vacuum system.

Electrostatic field between the cathode and anode ionizes the gas forming a glow discharge plasma at the workpiece surface.Positively charged ions of hydrogen and nitrogen are attracted by the negatively charged workpiece. They are accelerated by the electric field.The nitrogen ions move towards the cathode and bombard its surface where they dissolve and chemically react with the steel components (iron, chromium, aluminum, vanadium, molybdenum).

Most of nitrides formed in the surface layer (case) are iron nitrides Fe2N, Fe3N, Fe4N.

Hydrogen contained in the gas mixture is required for cleaning the metal surface from the oxides. Oxide-free and activated surface easily reacts with the nitrogen ions.

The steel workpiece is heated by the glow discharge plasma to a temperature 700-1200°F (370-650°C). Titanium alloys are treated at higher temperatures (up to 1600°F/870°C).

The temperature determines the speed at which the depth of the white layer (case depth) increases.Typically the case depth is within the range 0.004-0.025” (0.1-0.6 mm).The treatment time is commonly 10-50 hrs.

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Advantages of ion nitriding

Advantages of ion nitriding over ammonia nitriding are as follows:

Shorter (by 20-50%) treatment cycle. Better process control and automation. The process parameters (pressure, voltage,

temperature, gas flow, DC current) are easily controlled.

Higher surface hardness may be achieved due to lower process temperature (up to 1200 HV).

Better dimensional stability (lower distortions) due to lower process temperature and uniform heating.

Cases with uniform depth are formed even over parts with complex shapes.

Easier masking for selective nitriding.

Lower energy consumption due to lower temperature and shorter treatment cycle.

Reduced gas consumption.

Safer operation.

Lower environment pollution.

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Applications of ion nitriding

Ion nitriding is used for Case hardening of Alloy steels, Stainless steels, Titanium alloys.

Ion nitriding is effectively used when high surface hardness, good wear resistance, increased fatigue strength and toughness are required.

The following parts are processed by ion nitriding:

Cutting tools Forging dies

Drawing dies

Molds for Transfer molding and Compression molding of polymers

Machine and automotive parts subject to wear under friction (gear wheels, valves, lifters, cams, rocker arms, crankshafts)

Tempering

Tempering is a heat treatment operation involving reheating hardened steel to a certain temperature below the lower critical point (A1) followed by soaking and then cooling.

The steel structure after hardening consists mainly of martensite which is hard and brittle. Tempering is carried out in order to change the martensite structure and obtain a desired combination of strength and ductility.

The object of tempering is also to reduce the internal stresses caused by quenching.

Depending on the tempering temperature, the following stages of tempering take place:

Tempering at temperatures 300°F - 480°F (150°C - 250°C). The soaking time is commonly about 1-3 hrs. At these temperatures low carbon (0.25%) tempered martensite and fine dispersed carbides form. The internal stresses are partially reduced and some softening (by 2-3 HB) occurs at this stage.

Tempering at temperatures 570°F - 750°F (300°C - 400°C). Soaking time varies from 2 to 8 hours depending on the parts size. At these temperatures martensite transforms to trostite (very fine mixture of ferrite and cementite). Trostite is softer than martensite and more ductile.

Tempering at temperatures higher than 750°F (400°C) but lower than lower critical point(A1). Soaking time varies from 2 to 8 hours depending on the parts size. At these temperatures martensite transforms to sorbite (fine mixture of ferrite and cementite). Sorbite and trostite are principally similar structures differing only in the particles size. Sorbite is more ductile and less strong than trostite. This kind of tempering is used for the parts exposed to impacts.

If the tempering temperature is above 1020°F (550°C) strength decreases sharply without any notable increase of ductility.

Batch type furnaces ( either air atmosphere or liquid bath) are used for the tempering heat treatment.

Oil baths are widely used for tools tempering at relatively low tempering temperatures 300°F - 600°F (150°C - 315°C).

In order to prevent cracking the steel part should be preheated before immersing to hot oil.

Molten salt baths are used for tempering at temperatures 400°F - 1020°F (200°C - 550°C). Mixtures of sodium nitrate and potassium nitrate are suitable as the bath medium

Precipitation hardening

Precipitation hardening (age hardening) – strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution.

The precipitation hardening heat treatment involves the following stages:

Solution treatment

During solution treatment a part is heated to a temperature above the solvus temperature in order to dissolve the second phase in the solid solution.

The part is held at this temperature for a time varying from 1hour to 20 hrs. until the dissolving has been accomplished.

The temperature and the soaking time of solution treatment should not be too high to prevent excessive growth of the grains.

Quenching

Quenching is carried out in water, water-air mixture or sometimes in air.

Object of the quenching operation is obtaining supersaturation solid solution at room temperature.

Since the second phase retains dissolved at this stage, hardness of the quenched alloy is lower than after age precipitation, however higher than hardness of the alloy in annealed state.

Aging

Depending on the temperature at which this operation is carried out aging may be artificial or natural.

Artificial aging.

At this stage the part is heated up to a temperature below the solvus temperature, followed by soaking for a time varying between 2 to 20 hours.

The soaking time depends on the aging temperature (the higher the temperature, the lower the soaking time).

The aging temperature and the soaking time are also determined by the desired resulted combination of the strength and ductility of the alloy.

Too high aging temperature and time result in overaging – decrease of the strength and increase of ductility.

Natural aging.

Natural aging is conducted at room temperature and it takes a relatively long period of time (from several days to several weeks).

Precipitation hardening heat treatment is commonly used for the following alloys:

Aluminum alloys : Al-Cu, Al-Mg-Si, Al-Cu-Mg, Al-Zn.

Copper alloys : beryllium bronze,aluminum bronze, aluminum-nickel bronze, chromium copper.

Stainless steels : iron-chromium-nickel alloys with additions of copper, aluminum, titanium and niobium. The precipitation hardening stainless steels are either austenitic or martensitic.

Nickel alloys : nickel-copper alloys with additions of titanium, aluminum and iron.

Titanium alloys : titanium-aluminum alloys, titanium-vanadium alloys.

Cryogenic treatment of steelCryogenic treatment (tempering) is a processing of the material at a temperature below 80K (-315°F / -193°C) resulting in modification of its microstructure and improvement of its properties.

Effects of cryogenic treatment on steel microstructure Benefits of cryogenic treatment

Applications of cryogenic treatment of steels

Effects of cryogenic treatment on steel microstructure

Cryogenic tempering of a steel is carried out as a supplemental process following after the conventional heat treatment procedure (Hardening).

Hardening treatment comprises heating the steel above the phase transformation temperature (upper critical temperature), followed by soaking and then rapid cooling (quenching).

When steel is heated above the upper critical temperature, its structure becomes entirely austenitic, which transforms into martensite (supersaturated solid solution of carbon in α-iron) after quenching.Austenite-martensite transformation is never complete - a certain percentage of austenite is retained in the resulting microstructure. Austenite has face centered cubic (FCC) structure, which is denser than the body centred tetragonal (BCT) structure of martensite. The densities difference causes internal stresses in heat treated steels.Austenite is softer than martensite therefore high percentage of retained martensite decreases the steel hardness and wear resistance.

Cryogenic treatment results in the following effects of the steel microstructure:

Transformation of retained austenite into martensite; Internal stresses relief affecting most mechanical properties;

Precipitation of fine carbide particles (ETA-carbides) uniformly distributed in the martensite grains.

More homogeneous microstructure due to reduction of micro-voids (pores, cracks).

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Benefits of cryogenic treatment Wear resistance (due to higher hardness and the presence of hard ETA-carbides); Mechanical strength ;

Toughness ;

Fatigue strength (due to low residual stresses and homogeneous structure);

Creep ;

Low coefficient of friction (due to higher hardness and the presence of hard ETA-carbides);

Machining, grinding and polishing finish (due to no/little soft austenite);

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Applications of cryogenic treatment of steels Cutting tools for different machining operations: sawing, milling, drilling, broaching,

turning, slitting, shearing; Metal forming tools: dies, molds, punches.

High precision parts: gauges, guides, shafts;

Parts of high performance (sport) car engines and transmissions: crankshafts, connecting rods, piston rings, engine blocks, gear parts, camshafts.

Salt bath heat treatment

Salt bath heat treatment is a heat treatment process comprising an immersion of the treated part into a molten salt (or salts mixture).

Benefits of heat treatment in salt baths Compositions of salt baths

Heat treatments conducted in salt baths

Benefits of heat treatment in salt baths Fast heating. A work part immersed into a molten salt is heated by heat transferred

by conduction (combined with convection) through the liquid media (salt bath). The heat transfer rate in a liquid media is much greater than that in other heating mechanisms: radiation, convection through a gas (e.g., air).

Controlled cooling conditions during quenching. In conventional quenching operation either water or oil are used as the quenching media. High cooling rate provided by water/oil may cause cracks and distortions. Cooling in molten salt is slower and stops at lower temperature.

Low surface oxidation and decarburization. The contact of the hot work part with the atmosphere is minimized when the part is treated in the salt bath.

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Compositions of salt baths

No. Composition Approximate melting poing Work temperature range

1 NaOH 75%KOH 19%H2O 6%

284ºF (140ºC) 320-752ºF (160-280ºC)

2 KOH 50-60%%NaOH 50-40%

- 572-752ºF (300-400ºC)

3 KNO3 100% 639ºF (337ºC) 662-930ºF (350-500ºC)

4 KNO3 50-60%NaNO2 50-40%

275ºF (135ºC) 320-1022ºF (160-550ºC)

5 NaNO3 50-60%NaNO2 50-40%

293ºF (145ºC) 311-932ºF (150-500ºC)

6 KNO3 50-60%NaNO3 50-40%

437ºF (225ºC) 500-1112ºF (260-600ºC)

7 NaNO3 100% 698ºF (370ºC) 752-1110ºF (400-600ºC)

8

NaCl 10-15%KCl 20-30%BaCl2 40-50%CaCl2 15-20%

752ºF (400ºC) 932-1472ºF (500-800ºC)

9 NaCO3 45-55%KCl 55-45%

842ºF (450ºC) 1022-1652ºF (550-900ºC)

10 BaCl2 50%KCl2 30%NaCl 20%

1004ºF (540ºC) 1058-1652ºF (570-900ºC)

11 BaCl2 70-96%NaCl 30-4%

1112-1472ºF (600-800ºC) 1292-2282ºF (700-1250ºC)

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Heat treatments conducted in salt baths Quenching of steels. Quenching is rapid cooling from the temperature above A3

(upper critical temperature). Relatively slow cooling rate provided by molten salts prevents the work part from cracking and distortion.

Austempering. Austempering is the isothermal hardening method in which a part is quenched in a quenching medium (molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium after the complete bainite formation. Tha austempering temperature range is 400-750°F (204-399°C). Nitrate salts No. 4-6 are used for austempering treatment.

Martempering. Martempering is the isothermal hardening method in which a part is quenched in a quenching medium (molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium before the bainite formation. Martempering is performed at a temperature above the the temperature of martensite formation (austenite-martensite transformation), which is 400-480°F (200-250°C). Nitrate salts No. 4-6 are used for martempering treatment of most alloys. Sodium nitrate (No.7) a potassium nitrate (No.3) are used for martempering tool steels (hot-work and high speed steel).

Hardening. Hardening is performed at 1400-2300°F (760-1260°C) in chloride salts (No.8-11).

Nitriding. Liquid nitriding is the process of diffusion enrichment of the surface layer of a part with Nitrogen provided by a molten cyanide base salt (extremely toxic substance). The process is carried out at the temperatures 950-1075°F (510-580°C) for about 4 hour.

Carbonitriding. Liquid carbonitriding is the process of diffusion enrichment of the surface layer of a part with carbon and nitrogen provided by a molten salt containing 20-25% of sodium cyanide (extremely toxic substance). The process is carried out at the temperatures 1500-1580°F (820-860°C).

Carburizing. Liquid carburizing is the process of diffusion enrichment of the surface layer of a part with carbon provided by a molten salt containing 10-25% of sodium cyanide (extremely toxic substance). The process is carried out at the temperatures 1562-1742°F (850-950°C).

Solution treatment of Aluminum alloys. Solution treatment is the operation of heating the work park to a temperature at which the hardening second phase particles dissolve in the matrix. Solution treatment of heat treatable aluminum alloys is carried out at 900-1025°F (482-551°C). Fast solution heat treatment may be achieved by heating an aluminum alloy part in a molten salt bath. Nitrate salts No.4,5,6 are used for solution treatment of aluminum alloys.

Deep brazing. Brazing is a method of joining two metal work pieces by means of a filler material at a temperature above its melting point but below the melting point of either of the materials being joined. Dip brazing is a brazing method in which the work pieces together with the filler metal are immersed into a bath with a molten salt. The filler material melts and flows into the joint. Chloride salts with addition of reactive agents are used for deep brazing.

Cleaning. Polymeric contamination on metal parts surfaces may be effectively removed by immersion of the part into a molten salt. Polymers decompose and burn at the temperature of the molten salt. Mixtures of hydroxides and nitrates at a temperature within 650-950°F (343-510°C) are used for cleaning operation.

Furnaces for heat treatmentBatch furnaces

Car bottom furnace

Furnaces of this type have a movable bottom (car). The car goes out of the furnace and may be loaded or unloaded with treated parts.

The heating method may be either electric resistance or fuel/gas.

Car bottom furnaces are suitable for various heat treatment operations of large and heavy parts.

Bell type furnace

Furnaces of this type have a movable vertically heating bell and a stationary bottom with the treated parts.

The heating method may be either electric resistance or fuel/gas.

Bell type furnaces are suitable for coiled strip annealing and other heat treatments including operations in controllable atmosphere.

Vertical pit furnace

Furnaces of this type are used for heat treatment of shaft like parts (generator rotors, steam turbine rotors) which are loaded vertically through the top of the furnace.

The heating method may be either electric resistance or fuel/gas.

Continuous furnaces Belt furnace

Furnaces of this type have a mesh belt conveyor moving through a long tube like furnace.

The heating method may be either electric (resistance or induction) or fuel/gas.

Belt furnaces are suitable for heat treatment of relatively small parts.

Roller furnace

Furnaces of this type have heat resistant steel rollers moving the parts through a long tube like furnace.

The heating method may be either electric or fuel/gas.

Roller furnaces are suitable for heat treatment of sheets, tubes and other long parts.

Pusher furnace

Furnaces of this type have a pusher located at the furnace end and moving the parts through the furnace.

The heating method may be either electric or fuel/gas.

Pusher furnaces are generally used for heating parts before hot deformation.

Continuous strip annealing furnace

Coled rolled strip in uncoiled state passes through the long tube like or looped furnace with controlled reducing atmospere (commonly a mixture of Hydrogen and Nitrogen) preventing oxidation of the steel surface..

The heating method may be either electric or fuel/gas.

Grain structure

Grain is a small region of a metal, having a given and continuous crystal lattice orientation. Each grain represents small single crystal.

Grains form as a result of solidification or other phase transformation processes. Grains shape and size change in course of thermal treatment processes (for example recrystallization annealing). The normal grain size varies between 1µm to 1000 µm.

Grain structure of a solid is an arrangement of differently oriented grains, surrounded by grain boundaries.

Formation of a boundary between two grains may be imagined as a result of rotation of crystal lattice of one of them about a specific axis.

Depending on the rotation axis direction, two ideal types of a grain boundary are possible:

Tilt boundary – rotation axis is parallel to the boundary plane;

Twist boundary - rotation axis is perpendicular to the boundary plane;

An actual boundary is a “mixture” of these two ideal types.

Grain boundaries are called large-angle boundaries if misorientation of two neighboring grains exceeds 10º-15º.

Grain boundaries are called small-angle boundaries if misorientation of two neighboring grains is 5º or less.

Grains, divided by small-angle boundaries are also called subgrains.

Grain boundaries accumulate crystal lattice defects (vacancies, dislocations) and other imperfections, therefore they effect on the metallurgical processes, occurring in alloys and their properties.

Since the mechanism of metal deformation is a motion of crystal dislocations through the lattice, grain boundaries, enriched with dislocations, play an important role in the deformation process.

Diffusion along grain boundaries is much faster, than throughout the grains.

Segregation of impurities in form of precipitating phases in the boundary regions causes a form of corrosion, associated with chemical attack of grain boundaries. This corrosion is called Intergranular corrosion.

Crystallization

Some metallurgical processes involve phase transition.

The typical example of phase transition is crystallization.

Crystallization is transformation of liquid phase to solid crystalline phase.

There are two general stages of phase transformation (crystallization) process – nucleation and growth:

1. Nucleation

Nucleation is a process of formation of stable crystallization centers of a new phase.

Nucleation may occur by either homogeneous or heterogeneous mechanism, depending on the value of undercooling of the liquid phase (cooling below the equilibrium freezing point).

Presence of foreign particles or other foreign substance in the liquid alloy (walls of the casting mold) allows to initiate crystallization at minor value of undercooling (few degrees below the freezing point). This is heterogeneous nucleation.

If there is no solid substance present, undercooling of a hundred degrees is required in order to form stable nuclei or “seeds” crystals, providing following crystal growth (homogeneous nucleation)

Undercooling value determines quantity of nuclei, forming in the crystallizing alloy. When a liquid comes into a contact with cold and massive mold wall (chill zone), it cools fast below the freezing point, resulting in formation of a large quantity of stable nuclei crystals.

In order to promote the nucleation process, surface-active additives are used. They decrease interfacial energy of the nuclei crystals, causing formation of many more new stable nuclei.

2. Crystal growth

Number of stable nuclei per unit volume of crystallizing alloy determines the grain size.

When a large number of stable nuclei are present in chill zone of mold, fine equiaxed grains form. Latent crystallization heat, liberating from the crystallizing metal, decreases the undercooling of the melt and depresses the fast grains growth.

At this stage some of small grains, having favorable growth axis, start to grow in the direction opposite to the direction of heat flow. As a result columnar crystals (columnar grains) form.

Contrary to the pure metals, in alloys different type of undercooling takes place. It is called constitutional undercooling.

Constitutional undercooling

Since solubility of an alloying element in solid is lower, than in liquid at the same temperature, this element (solute) is rejected by the solidifying metal to the liquid phase, enriching the region of liquid adjacent to the crystallization front.

For the most of the alloys: the higher the concentration of alloying element in the alloy, the lower its liquidus temperature (temperature at which crystallization of the alloy starts).

Thus crystallization temperature of the liquid, adjacent to the crystallization front, rises with increasing the distance from the front surface. Therefore there is a layer of the liquid, where its temperature is lower, than its crystallization temperature. This is the region of constitutional undercooling (see the figure below).

Dendrites

If a protruding finger forms on the solidifying surface, its tip may reach the region of constitutional undercooling . In this case the protuberance starts accelerated growth, forming the main dendrite arms. Under certain conditions the same process may occur on the surface of the main dendrite arms, causing branching off the secondary arms and then arms of higher orders.

Process of solidification is considered in the article Solidification.

Imperfections of crystal structure

There are three conventional types of crystal imperfections:

Point defects

The simplest point defects are as follows:

Vacancy – missing atom at a certain crystal lattice position;

Interstitial impurity atom – extra impurity atom in an interstitial position;

Self-interstitial atom – extra atom in an interstitial position;

Substitution impurity atom – impurity atom, substituting an atom in crystal lattice;

Frenkel defect – extra self-interstitial atom, responsible for the vacancy nearby.

Line defects

Linear crystal defects are edge and screw dislocations.

Edge dislocation is an extra half plane of atoms “inserted” into the crystal lattice. Due to the edge dislocations metals possess high plasticity characteristics: ductility and malleability.

Screw dislocation forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part. It is called screw as atomic planes form a spiral surface around the dislocation line.

For quantitative characterization of a difference between a crystal distorted by a dislocation and the perfect crystal the Burgers vector is used.

The dislocation density is a total length of dislocations in a unit crystal volume. The dislocation density of annealed metals is about 1010 - 1012 m−². After work hardening the dislocation density increases up to 1015 - 1016 m-². Further increase of dislocation density causes crackes formation and fracture.

Planar defects

Planar defect is an imperfection in form of a plane between uniform parts of the material. The most important planar defect is a grain boundary. Formation of a boundary between two grains may be imagined as a result of rotation of crystal lattice of one of them about a specific axis. Depending on the rotation axis direction, two ideal types of a grain boundary are possible:

Tilt boundary – rotation axis is parallel to the boundary plane;

Twist boundary - rotation axis is perpendicular to the boundary plane:

An actual boundary is a “mixture” of these two ideal types.

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Grain boundaries are called large-angle boundaries if misorientation of two neighboring grains exceeds 10º-15º.

Grain boundaries are called small-angle boundaries if misorientation of two neighboring grains is 5º or less.

Grains, divided by small-angle boundaries are also called subgrains.

Grain boundaries accumulate crystal lattice defects (vacancies, dislocations) and other imperfections, therefore they effect on the metallurgical processes, occurring in alloys and their properties.

Since the mechanism of metal deformation is a motion of crystal dislocations through the lattice, grain boundaries, enriched with dislocations, play an important role in the deformation process.

Diffusion along grain boundaries is much faster, than throughout the grains.

Segregation of impurities in form of precipitating phases in the boundary regions causes a form of corrosion, associated with chemical attack of grain boundaries. This corrosion is called Intergranular corrosion.