Furnaces for heat treatment

Batch 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.

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.

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.

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)

Cryogenic treatment of steel

Cryogenic 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).

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);

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.

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.
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.

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.

Normalizing

Normalizing 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.

Annealing and stress relief

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.

Creep

Creep is a phenomenon of slow plastic deformation (elongation) of a metal at high temperature under a constant load.
The creep mechanism:
At low stresses the creep is controlled by the diffusion of atoms through the grain boundaries. At higher stresses the creep strain proceeds due to the dislocations movement.
The rate of creep is a function of the material, the applied stress value, the temperature, and the time exposure.
Considerable creep deformation, causing damage of machines and structures occur at high temperatures (about a half of the melting point measured in the absolute temperature scale). Therefore this phenomenon is taken into account in design and operation of heat exchangers, steam boilers and pipes, jet engines and other loaded equipment, working at high temperatures.
Soft metals (lead, tin) may experience creep at room temperature.
A typical creep behavior is presented in the diagram:
 
The initial strain is not time dependent and it is caused mainly by elastic deformation.
The first stage creep is characterized by relatively fast plastic deformation occurring at decreasing rate. During this stage resistance creep increases causing decrease the deformation rate.
The second stage creep occurs at a constant and relatively low deformation rate. This rate is used in the engineering design.
The rate of creep at the second stage depends on both the load (stress) and the temperature.
The third stage creep is associated with accelerated strain rate caused by decrease of the cross sectional area of the specimen (necking). This stage is finalized by the specimen fracture.
At room temperature creep is negligible at any stress below the yield point.
The quantity, which is used in precise design of machines and structures working at elevated temperatures, is creep strength.
Creep strength is a stress which causes a definite creep strain after a specified period of time at a given temperature.
Creep strength of a material is much lower, than its tensile strength.
If a large amount of deformation is tolerated rupture strength is used in design.
Rupture strength is a stress which causes a fracture of a metal after a specified period of time at a given temperature.
Creep strength and rupture strength are determined in stress-rupture tests conducted in [Tensile test and Stress-Strain Diagram|tensile test]] machines equipped with a furnace providing uniform heating of the tested specimens.
This machine records amount of strain at every moment after the test has started and until the specimen failure.