Nov.2023 20
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How to prevent distortion for case hardening
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On Google it says: "By applying the technology of Low Pressure Carburizing (LPC) and High Pressure Gas Quenching (HPGQ), heat-treat distortion can be significantly reduced. LPC is a case hardening process that is performed in a pressure of only a few millibars using acetylene as the carbon source in most cases."

When vacuum carburising (carburising in a vacuum furnace) was first introduced to the United States in the 1970's, the original concept was simply to provide an environmentally benign process using only propane as the carburising gas, as opposed to the traditional poisonous and dangerous mixture of carbon monoxide and hydrogen. The pressure range used at that time was just below atmospheric (about a few hundred millibars) to allow the use of a fan to move the carburising gas through the charge. The difficulties with this process were the control of the carbon, the control of the depth uniformity of the carburised layer and the reduction of the formation of carbon black.
 
The development of modern low pressure carburising processes using pressures in the millibar range began in the 1980's when a French lathe manufacturer promoted the technology, using lower pressures to reduce carbon build-up, improve the passage of carbon through the charge and even eliminate the need for a fan to agitate the carburising medium. The reason for this was simply due to the longer mean molecular free range. However this process development could not yet be widely accepted due to one drawback at the time. A serious limitation: the relationship between the surface area of the carburisation and the size of the effective zone of the furnace depended on the dispersion of the acceptable depth of the carburisation layer from part to part within a furnace.
 
The real industrial era of this new process, now called "Low Pressure Carburising (or LPC), started in 1992, when a furnace manufacturer designed a production line consisting of a few carburising chambers of limited volume (in order to reach the required production volume). A newer step forward was the introduction of acetylene gas as the gas of choice for propane at the end of the 90's, which significantly improved the handling of dense charges consisting of a large number of parts with complex geometries.
 
More than 600 low-pressure carburising chambers have now been installed worldwide, with most lines having four to six chambers, as well as an oil or gas quenching chamber. The distribution of the charge between the chambers varies according to the design of each different furnace builder, but always follows the same sequence: charge into an isolation chamber, evacuation, transfer of the charge under vacuum to the carburising chamber, carburising, transfer of the charge under vacuum to the quenching chamber as fast as possible, quenching, discharge through an isolation chamber or quenching chamber.
 
Not forgetting its other advantages, the LPC line is a cold-wall design with no fumes, no external heat radiation or gas emissions, advantages that were key factors in making the LPC concept very well known in the automotive industry for a short period of time.The LPC plant can be installed directly on the machine shop floor, which reduces the production cycle time and the process running costs significantly.
 
This success did hinder the development of LPC technology in the heat treatment subcontracting industry, with only a small number of installations in specialised heat treatment plants prior to 2000. In fact the LPC process can bring a number of key advantages to the heat treating subcontracting industry, as outlined below.

1. Principles of modern low-pressure carburising processes
 
All the hydrocarbon gases used so far for low-pressure carburising are shown in Table I, indicating in a very simple way the cracking products of the gas molecules on the surface of the steel under conditions of temperature and pressure. It is well known that methane molecules do not decompose under these conditions unless we use plasma to help, and glow discharges crack the molecules immediately above the steel surface. Therefore methane gas can be considered an inert gas in a pure LPC process, and only becomes active after plasma excitation, a process known as PA-LPC, "Plasma Low Pressure Carburising". However, unless there is a genuine need for plasma treatment (e.g. surface preparation activation, or localised anti-carburisation of the metal jacket), plasma low pressure carburising has been largely abandoned in view of the significant developments in low pressure carburising processes using propane and, more recently, acetylene, which do not require the complex furnace design that would result from the installation of a plasma unit. In addition, ethylene is no longer used due to its tendency to produce carbon black and tar in the cold zone of the furnace.
 
It can be said that about 95% of the processes worldwide are now using propane or acetylene (without plasma). The increase in the use of acetylene is due to the ability of the acetylene molecule to transfer carbon atoms to very large surface areas, to very complex geometries, such as long and/or blind holes. The difference between the carburising ability of the two molecules is that, in contrast to propane, acetylene (if operated under the right conditions) does not produce methane as an intermediate product, but rather produces radicals and molecules (e.g., vinyls), which are ultimately cracked into carbon and hydrogen.

2. Transmission component applications
 
    As already described above, the success of low-pressure carburising in the lathe industry is most immediate, due to the fact that the equipment can be installed in-line by the vehicle or component manufacturer for integration into the machining line. All these applications apply to transmission parts like shafts, pinions or other gearbox parts. In addition to the advantages mentioned above, two other important points have been taken into account by engineers and producers: the absence of intergranular oxidation and the reduction of distortion.
 
The first advantage has been reported from the very beginning. As a typical metallurgical difference from conventional atmosphere carburising, the low pressure carburising process uses only gas molecules without any oxygen atoms, which improves the surface hardness and fatigue properties of parts heat-treated to (or close to) their final dimensions without grinding or after carburising with only profiling. Reduced distortion due to the following factors is of even greater concern for gearbox parts:
 
-Better temperature uniformity (throughout the furnace, or from the root to the top of the gear teeth) The movement of carbon leading to changes in the carburised layer depends only on the internal geometry of the part.
 
-The use of high-pressure gas rather than oil for cooling is easier to realise in the vacuum-designed furnace quenching mode than in the conventional atmosphere-based carburising seal quenching. The proper regulation of pressure, speed and the characteristics of the cooling gas allow the cooling rate to be adjusted precisely to the desired value, thus meeting the corresponding property requirements of the part's core or penetration layer.
 
The limitation of course is that the cooling rate may not be fast enough for heavy parts made of low hardenability materials, or it may be achieved by using more expensive gases and equipment (helium, 20 bar, gas recovery and compression systems).
 
    It is now common to see specialist heat treaters offering low pressure carburising of small to medium batches of gearbox parts to automotive or component manufacturers, to achieve the same quality as they would in their own factories for high volume production. However, as heat treaters cannot easily influence the choice of material, it is often necessary to have both gas and oil quenching equipment in their production line. Gas quenching is used whenever possible, and oil quenching is chosen only when the hardenability of the material is poor for the weight and thickness of the part.
 
Figure 2 shows the loading of a gearbox shaft, during the preparation phase in a specialised heat treatment shop. The micrograph (top left) shows remarkable uniformity from the root to the top of the tooth.
 
The results of the deformation of these parts after quenching in a separate quench room using 20 bar nitrogen are comparable to those obtained previously by quenching in a salt bath. Obviously this is very favourable, but it should also be taken into account that there is no major diameter change in the shape of this shaft, nor is it very thin in section. Unless straightening of the part after quenching is acceptable, quenching in a salt bath is the only solution to control distortion after conventional atmosphere carburising.
 
In practice, the best solution (low pressure carburising + 20 bar quenching + straightening) is usually used for shaft parts with very sensitive distortion requirements, with the benefit of reducing to zero the risk of cracking and rejection caused by the straightening operation. Other heavy duty gearbox gear specialists report significant savings in grinding costs, with low pressure carburising reducing final machining costs from 25% up to 85% compared to the previous conventional process.

3. Advantages of low pressure carburising specific to the heat treatment subcontracting industry
 
     The previously presented examples of low pressure carburising applications in the subcontracting industry do not emphasise any clear advantages when comparing the specialised heat treating industry with self-processing users. However, if compared to a large scale self-processing application, the subcontracting business has more specialised (or at least stronger) needs.
 
3.1 Flexibility
 
Flexibility is certainly one of the most important speciality requirements for professional heat treaters. They require equipment that can not only be switched on and off according to the customer's needs (obviously this corresponds to peaks and valleys for self-processing users), but also allows for two consecutive cycles. but also to allow for process variations between two consecutive batches. In fact, the dreams of a professional heat treating plant in terms of flexibility can be realised with LPC: first of all, the vacuum technology allows the plant to be switched on and off as quickly as possible with the start or end of a job. The main thing is that the carbon transfer to the workpiece surface starts and stops with the injection of the carburising gas and is not affected by the state of the insulation (as opposed to conventional atmosphere carburising furnaces and their brick insulation). Subsequently a neutral hardening process can be followed immediately by a deep carburising process, both treatments giving satisfactory metallographic results respectively. The carburising temperature range (900 to 1050 degrees Celsius) is particularly useful if the heating capacity of the hot chamber can reach 1200 degrees Celsius, especially for tool steel hardening requirements.

3.2 Part-to-part deviations in the same furnace
 
This is another critical factor for specialised heat treaters who, in order to meet their customers' short lead times, have to treat the small orders of parts they receive from their customers in the same furnace by material. This is no problem at all for low pressure carburising. As shown in Figure 3, these different batches of machined parts can be easily treated in the same furnace as an atmosphere furnace.
 
    The only requirement is that the total carburised surface area is limited to a given value. The larger the carburised surface area, the greater the dispersion of the layer distribution (deviation of the layer between the parts in the centre of the charge and the parts at the edges).

3.3 Cleanliness
 
It is still a long way off for cleanliness requirements to be included in a contract. But even if a heat treating subcontractor is not required to include cleanliness requirements in the standard for each heat treated part, it is always an added bonus if the parts returned after heat treatment are as bright and white as they were before. Gone are the days when carburised parts had to be black and dirty (at least to prove they had been treated). Choosing low pressure carburising is the right answer: if properly controlled, the process will not damage the surface quality of the part at all or even change its colour. If we use high pressure gas as the quenching medium, the parts will come out of the furnace as bright and white as they were before they went in.
 
Therefore, even from a purely cosmetic point of view, a good cleanliness after a low pressure carburising treatment adds value. New standards have now been set in this area, which will put pressure on conventional atmosphere carburising processes. In addition, if we consider hydraulic or diesel injection system components, cleanliness is not only a cosmetic requirement for a good surface appearance, but also a very important requirement. Low pressure carburising combined with high pressure gas quenching is the ultimate answer.


 
If the carburised gearbox part is as shown in Fig. 2, then this limitation of the surface area for the low-pressure carburising process does not affect the quantity per furnace, in which case the limiting parameter is the weight or dimensions of the part. However, if the carburised part is a small part with a complex profile (e.g. like a spiral pinion for an automatic guidance system) or a small size part with many complex deep holes (e.g. like some of the components of a diesel injection system), the total surface area of the carburising process will be the key factor in determining the number of furnaces to be filled per furnace. Any attempt to exceed the maximum charge limit will result in a lack of carbon in the centre of the charge, or even no carbon at all in the centre of the part.
 
Another issue when discussing the application of low pressure carburising to commercially heat-treated parts is the possibility of handling large quantities. If acetylene (which is very effective for carburising in small spaces and for uniformity of the layer) is used, there is no problem as shown in Figure 4. From 15,000 small parts processed in one furnace, 30 pieces were taken at random from different layers for testing, and the detected deviation in the depth of the carburised layer was ±0.05mm (from 0.43mm to 0.53mm), which is lower than the deviation required by the standard (0.4mm to 0.6mm). The total carburised surface area is less than 3 square metres, which is far below the limiting carburised surface area requirement for equipment with effective dimensions of 900*600*600. Each part was uniformly carburised without any soft spots found.
 
Another important point that exists for a professional heat treatment plant is the impossibility to carry out many pre-tests to determine the layer depth, the surface hardness, and the surface carbon concentration. The parts he treats belong to the customer, and any unsuccessful trials lead to the question of who will bear the cost of scrap. And time is usually always compressed to a minimum. This means that the professional heat treaters must treat the parts immediately without any pre-testing and must be completely successful in the first furnace. The use of low-pressure carburising is very advantageous for professional heat treaters. This is due to the fact that the carbon transfer can be predicted and controlled very accurately (as it is based on known laws of physics) and the simulation software can guide the user very quickly and efficiently to the best process parameters for the carburising cycle.
 
The amazingly outstanding accuracy, predictability and reproducibility of this process is highlighted in Figure 5. This is an example of a new low-pressure carburising chamber using garden rings as an acceptance test. The cycle time of the process is determined by simulation software. The results are obtained by detecting the increase in weight of each ring after carburising. This parameter is well measured and is also a very good representation of the ability of the part to absorb carbon.
 
The y-axis of the graph shows the amount of change (in percentage) in the average value of all the measurements corresponding to the increase in weight of each ring. This average value is very close to the expected value given by the simulation software. The graph on the left shows where the measured rings were placed in the charge. It is surprising that the maximum deviation from the average value for the two rings located in the top corners of the charge (shown in the rightmost columns of the graph) is not within ±5 to 10%, especially since one ring shows an increase in weight of 35% compared to the target value. In fact the effect on the depth of the carburised layer does not show anything special, all rings are within the standard range of 0.6-0.8 mm. If this had been an atmosphere carburising furnace, no irregularities would have been reported and the equipment could certainly have been accepted.
 
However, this was a low pressure carburising furnace and further analyses and corrections were required. The final analysis showed that this was due to an unsuitable gas discharge between the furnace wall and the chamber, when the injection of acetylene (here used as a carburising gas) gas was finished, and during the diffusion phase the acetylene molecules were adsorbed in the chamber by means of the heated electrodes. Once the problem was solved, very satisfactory results were obtained, and the weight increase of all test rings returned to within ±5%.
 
The benefit of this experience was that it demonstrated that the use of low pressure carburising equipment must give outstanding results and that all the simulation software used to predict the results was sufficiently reliable to help guide the technicians in uncovering the problem and finding a solution.

4. Limitations of low-pressure carburising in subcontracting applications
 
Low pressure carburising, like any other process, has its own limitations. Here we only briefly focus on the pre-treatment of the surface of the part prior to the process, which is very critical for the heat treatment professional.
 
Low pressure carburising is very sensitive to several types of surface residues, such as alkaline detergent residues from pre-cleaning that are not completely removed prior to the carburising process. Some small rust particles on the surface of the part may cause a catalyst for the cleavage of acetylene molecules, resulting in the localised formation of carbon black on the surface. Small black spots not visible to the naked eye on the surface, which are usually limited, do not have a negative effect on the metallographic results.
 
More danger comes from boron-containing residues in some of the water-based coolants used for part machining prior to heat treatment. If these coolants are not properly eliminated prior to heat treating, the boron prevents carburisation during the low pressure carburising process, resulting in the formation of soft spots on the part surface. This soft spot formation on the part surface is random. But they are usually more likely to form in cavities where the boron containing material cannot be easily cleaned and may dry on the surface of the steel. One can imagine how dangerous this problem can be. Here we are confronted with an aspect that is very critical for the heat treating professional, because the professional heat treating plant does not control or rarely knows the condition of the coolant used prior to heat treating. The only way to combat the risk is to use the most advanced pre-cleaning equipment available for effective cleaning, which also includes good maintenance as well as measurement procedures. This may increase the cost of the cleaning process and affect the judgement of the global economic application of the low pressure carburising process.

5. Summary
 
The low pressure carburising process is now available as a very good alternative to conventional atmosphere carburising furnaces and has already proved to be very beneficial to the lathe industry for the high volume production of gearbox parts, with a number of component suppliers already installing a large number of multi-chambered low pressure carburising equipment into their machining lines.
 
The application of this technology in the subcontracting industry is not that far advanced, but the advantages of the low pressure carburising process itself, together with the vacuum based design of the equipment, have brought significant benefits to the heat treating professional, such as flexibility, good tolerance control for different furnace types and part sizes, good reproducibility due to the accuracy of the process control, ease of simulation of the process, and excellent surface cleanliness.
Because of these advantages, there is no doubt that the use of low pressure carburising in specialised heat treatment plants will soon reach a stage of rapid development.

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