Diffusion Surface Treatment Techniques: A Review
Surface treatment, also known as surface modification, surface engineering, and case hardening, can be divided into two distinctive groups: deposition and diffusion techniques. Deposition techniques are characterized as transporting a metallic substance from a source metal and depositing it onto the surface of another metal. These techniques include electroplating, hard coating (thermal flame spray), physical vapor deposition (PVD) and chemical vapor deposition (CVD). Diffusion techniques are further subdivided into two distinctive categories: thermochemical and thermal. Thermo-chemical diffusion techniques, namely nitriding, carburizing, carbonitriding, ferritic nitrocarburizing and boronizing are characterized as diffusing an element, such as carbon, nitrogen, sulfur, boron, and oxygen, into the surface of the steel by the application of the appropriate amount of heat, time, and the steel surface catalytic reaction. Figure 1 shows the process temperature range and process characteristics of different thermochemical diffusion techniques. Thermal techniques are those that modify the surface phases of steel containing sufficient carbon to allow the transformation from austenite to martensite when the appropriate amount of heat is applied to the immediate surface. This sometimes is known as phase hardening and is applied to processes such as flame, laser, induction, and electron beam.
Thermochemical diffusion processesNitriding
Nitriding is a low-temperature method used to diffuse nitrogen into the surface of the steel without changing the phase structure of the steel. The ferrite or cementite phase (depending on the carbon content of the steel) remains the same as originally; it does not change to austenite. Thus, there is minimal distortion because no phase changes occur in the steel. Traditional methods of nitriding, such as pack nitriding, gas nitriding, and salt-bath nitriding have been well documented.
Traditional gas nitriding, which has been favored by metallurgists and engineers for many years, now is being challenged by more recent techniques such as controlled nitriding, ion nitriding, enhanced ion nitriding and RF nitriding. Controlled nitriding and ion nitriding have gained acceptance and now are commercial techniques.
Controlled nitriding is a development of the traditional gas nitriding in which all of the process parameters are computer controlled including a gas panel for precise gas mixing and flow measurements, the process gas-analysis system, the process gas pressure within the process retort and process temperature and time. Process parameters are monitored on a continuous basis via computer and precise adjustments are made to the process systems to ensure repeatable metallurgy in relation to the load surface area and the load mass.
Ion nitriding, while not a new process, is becoming better understood by engineers and metallurgists. The technique was developed in 1932 by Wehnheldt and Berghaus as an alternative process to gas nitriding and salt-bath nitriding. It is only during the past 20 years that great strides have been made in controlling the plasma and reducing the potential for arc discharging. The capability of controlling arc discharging is a result of the development of a pulsed dc system. The system allows interrupting the power in time intervals of from 5 to 200 msec, rather than relying on a continuous dc power input. Conversely, the power-on time can be adjusted for a period from 5 to 200 msec.
The pulsed plasma dc system is the most commercially accepted in terms of ion-nitriding developments. To ensure process control and metallurgical repeatability, all system process parameters are controlled including:
- Process temperature, time, pressure, voltage and amperage
- Pulse on/off time
- Process gas (nitrogen, hydrogen and methane) flow rate
The pulsed plasma dc system was enhanced with the development of oxynitriding, wherein corrosion resistance of alloy steels is enhanced by the deliberate oxidation of the nitrided steel surface after the nitriding treatment. An advantage of the plasma nitriding system is precleaning of the workpiece prior to the nitriding process, although precleaning is not as intense as with gas nitriding. The precleaning method is known as sputter cleaning, or ionic bombardment. This can be likened to steel-shot blasting, but blasting of the workpiece surface is done using gas ions as opposed to steel shot-the surface of steel is almost atomically clean.
The net result of precleaning through ionic bombardment is that the steel surface is prepared for nitriding in a more efficient manner. Thus, the total process time is reduced by as much as 50 to 60% over that of conventional gas nitriding (depending on the case depth requirement, process temperature selected, and steel being treated). This is particularly evident on shallow case depth requirements. For example, a case depth requirement of 0.008 in. (0.2 mm) using gas nitriding requires a total process time of approximately 16 to 20 hours depending on the surface area of the load. By comparison, the same surface area could be treated using pulsed plasma nitriding in a total process time of approximately 8 to 10 hours, floor to floor.
Another development for use with the pulsed plasma nitriding system is surface deposition after the nitriding treatment. This allows nitriding to be carried out in the ion nitriding chamber followed by a surface deposition treatment, such as depositing titanium nitride.
Plasma-assisted ferritic nitrocarburizing
Plasma-assisted ferritic nitrocarburizing is a further development of the gas ferritic nitrocarburizing process. The process takes advantage of the plasma generation system and works in exactly the same manner as the plasma-assisted nitrocarburizing process. The difference in the processes is that carbides and nitrides are formed in the immediate surface of the steel together with the formation of epsilon nitrides, the dominant phase in the immediate surface, which results in a hard abrasive resistant surface. This treatment usually is applied to low alloy steels that require a hard abrasion resistant surface.
The carburizing process has developed from the earlier pack charcoal method to gas carburizing-now the most popular method of carburizing. A problem with gas carburizing is the control of the gaseous medium within the furnace to generate the correct amount of carbon monoxide (CO). If this is not controlled accurately, carbon dioxide (CO2) can begin to form, which leads to scale formation and oxide formation on the surface of the workpiece. Oxygen can then diffuse into the surface and along grain boundaries, thus forming grain-boundary oxides, which can lead to microscopic crack propagation in the carburized surface if not controlled.
Vacuum carburizing is a clean method used to introduce carbon into the surface of the steel and also prevents grain boundary oxidation. Vacuum, or low-pressure, carburizing is carried out in a vacuum furnace (figure 2) at pressures below that of normal atmospheric pressure. The principle of carburizing is exactly the same as that of the gas carburizing process, the main difference being the use of subatmospheric pressure.
Vacuum carburizing is somewhat sensitive to gas flow-control requirements. Another problem is the inability to measure the gaseous activity within the process chamber. The conventional gas carburizing process allows measuring the gaseous activity within the furnace process chamber using the shim test method, dew point test method, CO/CO2 test method, and oxygen probe.
The vacuum carburizing process provides only empirical data from which to develop process parameters, which can make the process difficult to repeat with consistent results. Generation of soot can easily occur if control of the process gas is not maintained in terms of flow and volume. The soot typically migrates to furnace cold spots (at power feed throughs and the inner water jacket). If the soot is not cleaned out frequently, soot build up at the power feed through can cause electrical shorting, which can make the furnace troublesome.
Pulsed plasma carburizing is the state-of-the-art carburizing process. Thus, as is with plasma nitriding, the workpiece is at cathode potential and any sooting that might occur migrates only to the cathode, which is the workpiece. The problem of gaseous control still is present, but sooting only occurs at the workpiece. Development work has been conducted in the design of a system to control gas chemistry based on light spectroscopy. It is necessary for the observation unit to "see" the plasma glow seam. A signal is generated and transmitted to a PC that both monitors and controls the gaseous activity within the process chamber through a process control unit in conjunction with the plasma generation system.
High-temperature carburizing is another accepted development of plasma carburizing technology (as it also is with vacuum carburizing). Conventional gaseous carburizing has been limited by the materials of construction of the process furnace. Capital investment might be prohibitive if modern gas carburizing furnaces were constructed from expensive high-performance materials.
Plasma carburizing opened the door to take advantage of higher processing temperatures than those temperatures typically encountered in gas carburizing. It now is possible to use process temperatures as high as 1900 - 1950F (1040 - 1065C) instead of carburizing at conventional temperatures to 1700F (930C). The net result of the higher temperatures is that the carburizing rate is considerably faster. For example, a total case of 0.040 in. (1 mm) at temperature of 1700F requires approximately 4.5 hours compared with 45 minutes at 1900F. It should be noted though that while faster diffusion rates are possible at higher temperatures, the potential for distortion while holding at the higher temperature also is much greater. However, the holding time at the higher temperature is considerably shorter. There also is the question of grain growth occurring at the higher process temperatures, although studies show that the grain growth that occurs is no greater than the grain growth that occurs using conventional process temperature in gas carburizing.
Note that to harden the carburized case, it still is necessary to cool down from the higher process temperature to the appropriate case austenitizing temperature before quenching.
A further development of the high-temperature carburizing process is combining the technology with high-pressure gas quenching using blended mixtures of nitrogen and helium gases. Vacuum furnaces equipped with high-pressure gas quenching capability are shown in Fig. 3 and 4. The quench gases can be blended to suit the steel and the part configuration, thus reducing, but not eliminating, distortion. The results obtained on alloy carburizing steels are comparable to those obtained using gas carburizing. However, plain carbon carburizing steels do not quench out well using the blended gas system.
The overall result of plasma carburizing in conjunction with high-pressure gas quenching is that the treated part is extremely clean and, therefore, does not require any post cleaning. There is little difference in the appearance of the part surface before and after high temperature carburizing and high-pressure gas quenching.
Carbonitriding also can be accomplished in either vacuum or plasma conditions. The only additional gas necessary for vacuum carbonitriding is a small, precise flow of ammonia (HN3) into the process chamber. Plasma-assisted carbonitriding requires a small amount of nitrogen as the additive gas to ensure the formation of surface nitrides. Plasma-assisted carbonitriding is a faster process than conventional gas processes. The surface of the treated part is extremely clean and does not require any post cleaning.
Boronizing is a method used to diffuse the element boron into the surface of steel to form hard boron carbides and other complex compounds. Two basic boronizing techniques are the pack and paste methods.
In the paste method, about a 0.25 in. (6 mm) thick boron-rich paste is "plastered" on to the surfaces to be boronized. The part is placed inside a steel box having a loose fitting lid and the box is placed inside a furnace. The process temperature selected is dependent on the austenitizing temperature of the particular steel grade, and the resulting case depth depends on the time at temperature. The hardness values achieved using this method are in the region of 1700 VPN (Vickers pyramid numeral).
Pack boronizing is very similar to that of pack carburizing in terms of methodology, except that a granulate rich in boron is used instead of using a carbon-rich granulate as in pack carburizing. The part is packed into the boron-rich granulate in a steel box and sealed using a loose fitting lid. The box is placed inside a furnace at the appropriate austenitizing temperature for the particular steel grade.
Boronizing is not a widely used procedure although the process has been used for the past 25 years. Development work using a reactive gas under plasma conditions with vacuum-related technologies has been moderately successful.
Thermal diffusion processesFlame hardening
Flame hardening is a technique that uses the combustion of a hydrocarbon gas with oxygen or air to generate a high-intensity, high-temperature flame, which is directed at the surface of the steel workpiece. The part temperature can be successfully and accurately controlled to a very precise, tight tolerance. Temperature measurement is accomplished using optical pyrometry, infrared sensing, and direct thermocouple on the part.
Laser heat treatment
Laser heat treatment of steel is a relative newcomer to the surface modification group of treatments. Charles Townes and Arthur Schawlow first discussed laser theory in 1957, and Theodore Maimanin built the first unit in 1960, after which laser heat treatment of steel eventually became a commercially accepted process.
A simple laser system comprises a light-generation unit, a lens that is adjustable in terms of the manipulation of the focal length and a work-positioning station. After the light is generated, the focal length of the light can be manipulated to burn, cut, heat, and join simply by moving the lens.
The focal length is adjusted to the point where heat is generated on the immediate surface of the steel when using a laser for surface treatment. This causes rapid heating of the steel to the austenitizing temperature of the particular steel grade. Upon reaching the austenitizing temperature, the mass of the steel causes a heat sink, which is fast enough to transform austenite to martensite without the need for a liquid quench medium.
Application of the laser system to heat treat steel is used to great effect in small, confined areas that are inaccessible using an induction coil.
Induction hardening treatment
Induction hardening is a method used to heat the immediate surface of a workpiece using electromagnetic induction to raise the temperature of the steel into the austenite region followed by rapid cooling to transform the surface austenite to martensite. This is accomplished by inducing electrical currents (eddy currents) into the immediate surface of the workpiece. The eddy currents dissipate energy, and as a result, cause a heating effect. To accomplish this effect, the workpiece must be located within a heating coil, the design of which is determined by the required heating pattern. The flow of alternating current through the heating coil generates an alternating magnetic field that cuts through the workpiece; the alternating magnetic field infuses the workpiece with eddy currents that heat it.
The primary advantage of induction hardening is the ability to control the area that is to be treated. Other advantages include rapid surface heating, no surface scale, high production rates and considerable energy savings.
Major developments that have occurred in the field of induction heating are the development of more efficient power generation systems and great advances in coil design to intensify the surface power. Advances also have been made in the area of mechanical handling of the workpiece.
With the exception of processes like boronizing, the TS process, other salt-bath surface treatments, and flame, induction, laser, and electron-beam processes, surface-treatment processes generally are moving in the direction of vacuum-related technology. The other technology being recognized as an excellent processing tool is plasma technology, and in particular, pulsed dc plasma techniques. The technology is based on the ionization of a reactive gas in conjunction with the activation of the steel surface by ionic bombardment and other techniques of preparation. Industrialists are always looking for more cost effective processing techniques with the present high cost of energy and process gas, and the vacuum-related technologies in combination with plasma techniques offer a solution to this problem. Even with the high cost of investment, the savings in process time and operating costs can outweigh the investment cost with a price per pound comparable to those for the more conventional techniques. In addition, the requirement of less process effluent, not only from the process, but also from post-cleaning operations, as well as the need for faster process times are beginning to influence the process selection toward the use of vacuum/plasma-related process technologies. These techniques also produce more consistent and repeatable metallurgy.
Plasma technology also is beginning to influence thin-film surface deposition techniques, which traditionally have used the high vacuum, line of sight PVD system. Today, with plasma-assisted CVD techniques, there is increasing use of low-temperature processing using rough vacuum as opposed to high vacuum, and without relying on line of sight.
An increased understanding of these process techniques will enable metallurgists and engineers to meet the increasingly stringent demands placed not only on manufacturing methods, but also on performance demands of industrial parts in terms of wear, corrosion resistance, fatigue life, and impact strength.
SIDEBAR: Toyota diffusion process
A process known as the TS ProcessR is becoming a well-accepted process in terms of creating a wear resistant surface barrier, as well as providing resistance to scuffing, seizure, corrosion and allowing a considerable reduction in operating lubricants on tool steels. The salt-bath process uses a borax base, and forms combinations of vanadium, chromium and niobium carbides in the immediate steel surface. The steel part is dipped into a bath of molten borax containing the carbide-forming constituents that form a wear resistant layer in the surface of the steel. Very careful control of the chemical composition of the salt bath is necessary to ensure the success of the process.
The process is carried out at temperatures between 1550 and 2000F (845 and 1095C) at varying treatment times. Process temperature selection is based on the austenitizing temperature of the particular steel grade. As with other diffusion processes, the resulting surface metallurgy is a function of temperature and time. In the process, a coating is deposited on the part surface as a result of immersing in the salt bath, and coating diffusion into the substrate surface results in an atomically bonded, high-density surface layer to a depth of 8 to10 mm thick. The success of the process relies on the carbon content of the steel to form carbides in the immediate surface of the steel. Surface preparation prior to and after the procedure is of paramount importance to the service life of the treated part. The process is finding great favor in stamping, pressing, forming and drawing operations. Typical final surface hardness values on most tool steels are between 3000 and 3500 HV. Significant improvement in tool life is reported to be up to ten times the normal life expectancy.
While traditionally a salt-bath process, the TS Process in recent years has become an atmospheric process using fluidized-bed techniques. The TS Process is licensed by Toyota (Japan).