By Steve Harper and Dr. Tohru Arai
— Reprinted with permission from Die Casting Engineer, March/April 2000.
Carbide coating by TRD (Thermo-Reactive Deposition and Diffusion) can ensure very high adhesion strength of the carbide layers onto substrates due to the nature of the carbide formation mechanism. Carbide coatings made by this method have very good resistance to erosion and corrosion against molten cast metals.
Therefore, carbide coating by TRD has large potential to prevent failures of mold components used in die casting and other casting methods and to provide tremendous benefits to the die casting industry. In fact, many field test results recently obtained are accelerating the wide application of the process in the American die casting industry as in Japan, where the process has been successfully used since the mid-1970s.
TRD coating is usually carried out at temperatures similar to hardening temperatures for steels; for example 1877°F (1025°C) for H-13. Steel substrates are quench-hardened during cooling from the coating temperatures and followed by tempering. Therefore, distortion may occur. Overcoming distortion was the key point for successful application of the process. Working surfaces of the molds must meet required dimensional specifications on completion of the coating operation since the coated molds are put into use without a finishing process. Carbide layers formed in the TRD process are too thin to be polished off for dimension control.
Distortion in application of high temperature coating processes such as TRD and CVD is often overstated by recalling the distortion in ordinary hardening of steels. People often expect the degree of distortion to be similar to that in ordinary hardening of die molds based on past experience. This thinking is not correct. Ordinary hardening is done in most cases on the premise that the articles are to be ground to the finish size with grinding allowance large enough to get the finish size. This premise lessens the incentive to heat treaters to minimize distortion. Equipment and procedures are well organized and extra care is usually taken in the coating application to minimize distortion. Furthermore, ordinary hardening is applied to annealed steels while TRD coating, in most cases, is applied to already hardened steels. This brings about a good effect on dimensional change due to a smaller difference in microstructure change before and after TRD processing. Thus, the coating has been successfully applied to very close tolerance tools with tolerances of some µm in diameter. Following is an explanation of causes and countermeasures for distortion and some examples of results obtained on die casting molds.
Criteria of distortion
Dimension (size) change: Symmetric change and shape of tools remains unchanged (“a change of size without a change of shape”). This includes size difference between the edges and in the center of the faces.
Deformation: Non-symmetric and shape is changed (“a change of shape and size”).
Most tools change shapes to barrel or spool, more or less, depending on the shape and size of molds, and the coating condition. This is usually symmetric and stays in the category of dimension change.
Deformation becomes evident by curvature of long axes of slender tools and non-symmetric-shaped tools. Another typical one is out-of-roundness of ring-shaped tools.
Dimension of molds after TRD coating
Dimensions of molds will be changed by the following two reasons:
- Buildup of carbide on substrate
- Dimensional change of substrate material before and after the coating operation.
The dimension of tools after the coating, therefore, can be shown in relation to the initial dimension:
Da = Db + 2 Tc + ΔD
Da ; Dimension after coating
Db ; Dimension before coating
Tc ; Thickness of coatings
ΔD ; Dimension change of substrate
Carbide layer thickness, Tc, is usually selected from 0.0002 to 0.0005 in. (5 to 12 µm) in die casting application regardless of shape, size and kind of mold components to be coated. Tc can usually be small enough to not be a concern. ΔD can be much larger than Tc in the case of large cores, large core pins, etc., depending on various factors and should be a more serious concern than Tc. Under-sizing or over-sizing to the targeted final dimension can compensate for the possible dimensional changes. Therefore, scattering of Db, Tc, and ΔD should be of more serious concern rather than their individual values.
Causes of the dimension change and related factors
Tc is controlled mainly by TRD bath temperature, immersion time in the bath, and bath control and has a very minor effect. Scattering of chemical composition and microstructure of substrate steels can change the thickness of carbide layers through scatter of carbon content in the matrix phase (austenite) at the coating temperatures. However, this problem is not serious in H-type steels widely used in die casting since they contain relatively small carbon and alloying elements. Therefore, scattering of Tc can usually be out of consideration in die casting applications.
ΔD and its scattering can be affected by a number of factors related to the shape and size, substrate materials, heat treating condition, and coating condition. Volumetric change of the substrate by change of microstructures of the substrates is the primary reason for ΔD. That is to say, from pearlite to martensite and retained austenite, (in the case TRD on the un-hardened molds), change in amount of austenite (in the case of TRD on hardened substrates), and tempering of martensite. However, ΔD is not isotropic.
The size and shape of molds highly affects the difference in temperature between the surface and core during hearing or cooling which produces thermal stress and transformation stress. The barrel- and spool- shaped changes are determined not only by the values of these stresses but also the sequence in time by which the transformation stress is added to the thermal stress. Temperature differences and stresses will also be induced by change of thickness within a mold. Larger size, higher temperature, and larger rate of heating and cooling will make ΔD more considerable.
The banded structure in forged and rolled steels, which is large amounts of carbide particles in a line parallel to the rolling direction, make the expansion and construction heterogeneous. This carbide alignment usually leads to larger size movement in length than in diameter, thickness and width. The banded structure is determined not by the size of steel stocks but by steel making processing, including ingot size, forging ratio, and rolling direction etc.
Size change of tools can occur even at room temperature by transformation of “retained austenite” (high temperature phase of steel) to “martensite” (the phase caused by quench-hardening of steel). Except for tools that need very strict dimensional tolerance, this change of size is usually negligible.
In some cases, some factors are very serious and others are small and negligible.
Causes of deformation and related factors
Distortion is formed by the following reasons:
- Non-uniform heating and cooling during high temperature coating and heat treatment
- Non-uniform micro-structure in substrate material
Residual stress produced during tool making, such as machining and grinding.
Creep by gravity during high temperature coating and heat treatment.
Most heating and quenching equipment is not likely to make uniform heating and cooling within tools. The complicated shape accelerates this tendency. Not enough spacing between molds, or molds and basket components also results in differences of the heating and cooling rate. The heterogeneous microstructure in forged and rolled steels gives rise to different transformation temperatures and the time lag in transformation within a mold leading to deformation. Improper machining conditions bring about residual stress in molds, and relieving of the stress at high temperature can cause deformation, if heating rate is high. Steels have very small yielding stress at high temperature. The loading of self-gravity easily results in deformation such as warpage of long slender tools loaded in the baskets in such a way that the end of tools arc placed on the basket bottom, or disk shaped tools placed on a plate with bend, for example.
How to get TRD coated molds with minimum distortion
Following are some countermeasures to ensure minimized distortion:
- Select good type of substrate materials
- Select premium quality materials
- Cut out molds from steel stocks in consideration of rolling direction
- Machine and grind under proper condition to minimize residual stress
- Make molds with tight dimensional control
- Apply under- or over-sizing on targeted size
- Harden molds under precisely controlled conditions
- Apply TRD coating under precisely controlled conditions
- Control movement and deformation by selection of proper tempering conditions
- Apply deformation correction after TRD coating
- Design mold with consideration of finish grinding on non-working surface to ensure extremely tight tolerance.
As mentioned before, the H-type steels are basically good materials. It is, however, highly recommendable to use steel stock produced by the same steel maker since there is some scattering in quality, difference in smelting method, forging ratio, and scattering of chemical composition etc., between steel makers and even between steel stock made by the same steel maker.
ΔD and scattering is usually greatest in the direction parallel to the rolling direction. Therefore, it is recommendable to machine molds so that the critical dimensions are transverse of the rolling direction. Large scattering will be produced if this could not be ensured each time of repeated making of the same type of molds.
All molds requiring close tolerance should be made with high accuracy dimensionally since any kind of additional processing can increase the dimension range. As shown later, TRD coating can expand the dimension range: Max.–Min., about 0.0005 in. (13 µm).
We can obtain actual dimensional change values through repeated application to specified molds. Accumulating this data provides us the information on how large a dimension change will occur on the specified molds, as shown in the next paragraph. The targeted dimension can be obtained by making molds under- or over-size by these values. It should be kept in mind that these values vary more or less with change of all other factors related to distortion.
Scattering in the hardened condition should be minimized since ΔD is caused by the difference in microstructure in the substrate between before and after TRD coating. We have found through the TRD coating business that there is very large scattering of the hardness in the molds sent for coating, including standard core pins in market as exemplified in Figure 1a and Figure 1b. This large scattering in hardness is probably caused by the use of large vacuum furnaces where a large number of molds were set in a basket without consideration to ensure uniform heating and quenching on each mold. It is recommended to apply TRD coating onto finished molds without hardening, if the hardening condition cannot be well controlled. Applying TRD coating on un-hardened molds can decrease the probability of deformation. Slender and long pins are also likely to bend in the preliminary hardening operation. Substantial bending will be produced if these pins were mechanically corrected for the bending after preliminary hardening due to the residual stress generated by such bending correction.
Hardness of pins, as received and after the TRD coating—pins received from a die caster in a day.
Hardness of pins, as received and after the TRD cooling—pins received from a die caster on 28 different days.
The TRD coating operation should be done under well controlled conditions. To make 0.00023–0.00046 in. (6-12 µm) thick carbide coatings on H type steels, TRD coating is carried out by immersion into a molten borax salt bath at around 1877°F (1025°C) for 4–10 hours. The temperature of molds, as is the nature of molten salt, is very close to the temperature of salt, which can be easily controlled. The molds are taken out of the bath after a specified time (ΔD is not sensitive to immersion time) and quenched into a salt bath at 1000°F (540°C) followed by air cooling, after being kept in the bath long enough to equalize the temperature between the center and surface of the molds. This quenching is the best way to be industrially applied to minimize both dimensional change and deformation. Bad effects of other molds on uniform heating and cooling of molds should be minimized by well considered setting in a basket. The distortion problem is more remarkably affected not by the temperature and time but by uniformity in heating and cooling, which can be considerably influenced by the worker's procedures such as direction of molds in a basket, the space between basket components and other articles to be coated in the same basket, etc. Proper loading configuration is a significant factor in assuming the uniformity of heating and cooling of molds.
After the coating and simultaneous hardening of the substrate, tempering is carried out under the following condition: 1100–1125°F (590–620°C), 2 hours, 2–3 times.
The tempering operation makes dimensional changes by the tempering effect on the martensite phase produced during quenching, and decomposition of retained austenite to martensite/bainite. The dimensional change produced by tempering can be altered with tempering temperature, time and repetition of tempering, while keeping substrate hardness within acceptable values to die casting applications. Thus, precise control of dimension can be done by changing tempering conditions.
Slender and long pins are susceptible to bending. The residual stress formed during heat treating, correction of bend, grinding, etc. should be minimized. Stress relief annealing before TRD coating is recommended. Mechanical loading and local heating can be applied to correct the bend after TRD processing.
Add a grinding allowance on the non-working mating face requiring close tolerance and grind to the targeted dimension after TRD processing. This is widely applied since it is an easy way to ensure tight tolerance. However, it takes more time and adds cost in mold making and should be the second choice.
Use of good material, use of under- or over-sizing, cutting out molds in consideration of rolling direction, well controlled machining condition, and well controlled heat treating and TRD coating are key factors for eliminating distortion.
However, TRD coaters or mold makers can only do some of these things. The mold designer, mold makers, steel makers, heat treaters, and TRD treaters should be equally responsible for the problems of distortion. Whenever dimensional tolerances are critical, close liaison between each is highly required. It can be said that minimizing distortion is a matter of will.
Examples of distortion
The first example shows results of an experiment which was carried out by using H13 steel un-hardened rod specimens 0.513 in. (13 mm) dia. and 3.82 in. (77mm) long. Thirty specimens were cut out from long steel stock and TRD coated in three different loads, ten specimens each per load in a production bath. The specimens were located in the bath so that they were distributed uniformly over the whole effective space in the bath.
Two major causes for dimensional change namely, the scattering in steel quality and in preliminary hardening were eliminated in this test. Hence, dimension change and its standard deviation were negligibly small; average 0.00066 in. (8 µm) and 10.00009 in. (02.5 µm) for OD and 0.0001 in. (2.5 µm) and 0.00079 in. (20 µm) for length, respectively (Figure 2 a & b). The results suggest that TRD coating can be applied to un-hardened pins and, in fact, has been applied in Japan.
Changes of (a) OD and (b) length obtained on un-hardened small rod specimens of H-13 in an experimental test.
A similar test was done on relatively large round specimens simulating an actual production core insert. There was no preliminary hardening but three specimens were cut out from three different steel stocks of premium H13 for a total of nine specimens. One specimen was picked out from each of the three steel stocks, and were TRD coated in a load to distinguish the effect of scattering in steel stock and TRD coating operation.
The effects of steel stock and the loads were not observed and standard deviation values were very small, especially in diameters 0.0001 to 0.0004 in. (2.5 to 10 µm) for ID, 0.0003 to 0.0004 in. (8–10 µm) for OD, and 0.0008 in (20 µm) for height. The specimens were TRD coated again, without annealing, under the same condition. The size change and scattering in this test seem to be almost the same as those in the application of TRD coating on the specimens preliminarily hardened under the condition very close to that of TRD. The standard deviations obtained were also small. The dimension changes are different between the coating onto un-hardened specimens and Re-TRD coating.
Figures 3a and 3b exhibit the relation between OD dimension or length and change of OD or length which was obtained with large numbers of pins from a number of customers. Average change of shank OD is not sensitive to change of OD and is only 0.0002–0.0008 in. (5–20 µm) in the most popular size of pins; 0.2–0.6 in. (5–15 mm) OD. Very similar values were obtained to point OD. Length changed more remarkably, but was about 0.004 in. (100 µm) in pins with less than 6 in. (150 mm) length. The trend lines for average change of OD and length can be used to determine the recommendable over- or under-sizing values, if it is necessary.
However, scattering of dimension change should be considered here. It is presumed in this case that the all factors related to distortion problems were not necessarily well controlled in pin making procedures. Heat treating was done by a number of different companies under different conditions, resulting in a wide range of hardnesses, as shown before. Another big problem is the uncertainty of steel type. Notice of steel type from die casters is not necessarily correct and there may be a large probability of mixing different types of steels in the results. Some pins were possibly nitrided or carburied. These problems cannot be completely eliminated in practical applications in the die casting industry. Therefore, the results in Fig.3 should be considered to be normal in applications to the die casting industry.
Change of (a) OD and (b) length of pins mode of H-13 in relation to OD and length.
Bending of pins was measured on two long pins. A pin 7 in. (180 mm) long with 0.4 in. (10 mm) shank diameter and long tapered portion bent about 0.003 in. (0.08 mm). Another pin 7 in. (180 mm) long with 0.6 in. (15 mm) shank diameter and short taped working area bent 0.0016 to 0.0024 in. (0.04 to 0.06 mm). These bendings were successfully corrected by pressing.
Deterioration of out-of-roundness was evaluated in several examples. Out-of-roundness of the un-hardened round core specimens increased only 0.00015–0.00047 in. (4–12 µm) in both ID and OD. Those of hardened round core specimens showed only small increases: 0.00004–0.0004 in. (1–10), and 0.00008–0.00075 in. (2–19 µm).
Range of Diameter and Length
The relation between the range (maximum–minimum) of diameter and length were investigated, in the core pins before coating and those after the coating. The size range can increase by max. 0.00067 in. (17 µm) in diameter and max, 0.0018 in (40 µm) in length. Making the molds with minimized scattering of dimension should be the first step to satisfy the tight dimensional tolerance.
Application to die casting molds
As for TRD application, benefits similar to those obtained in Japan were reported in the USA two years ago. The application in American die casting is now being accelerated and a large number of pins and cores have been put into production reporting satisfactory results. Application on various types of pins including squeeze pins ranging in size from very small pins such as 0.157 in. (4 mm) in shank diameter to large pins as 1.57 in. (40 mm) in shank diameter and 19.7 in. (500 mm) long. Some types of core inserts with very complicated shapes, and sprue cores with 6 in. (150 mm) in diameter have been successfully used. Substrate steels are usually H-13 and other H type steels. Modified high speed steels are rarely used.
It seems to be commonly believed that high temperature coating processes cannot be applied to dimensionally tight products. This is not necessarily true. Punch makers in Japan have been making standard type punches for metal stamping with 0.00008 to 0.00012 in. (2 to 3 µm) tolerance in diameter and less than 0.0002 in. (5 µm) in bend for more than twenty years. The results of Example 1 and others suggest that die casting pins with similar tolerances can be made without difficulty by well-considered procedures. It cannot be expected to industrially realize similar tight tolerances for some large cores, especially with complicated shapes. However, the examples of actual applications shown in the previous paragraphs suggest that even relatively large cores can be successfully TRD coated, satisfying the required tolerance.
Unfortunately, a number of factors relate to distortion problems, most of which are out of the die caster's specialty. It should be emphasized that to easily control all these factors, the die caster should keep in close contact with steel makers, heat treaters, tool makers and especially with the coaters if he wants to apply the high temperature coating process to tight toleranced molds. The extra effort will be worthwhile and shown through cost savings.
This article, Diffusion Carbide Coating for Distortion Control was originally published and copyrighted in 2000 by the North American Die Casting Association (NADCA) in Die Casting Engineer, in March 2000. It is published here with the permission of NADCA and may only be republished with the permission of NADCA by contacting firstname.lastname@example.org. NADCA is the worldwide leader in stimulating growth and improvement in the die casting industry.