Best substrate materials for tools with tight dimensional tolerances in application of the high temperature TRD coating

Introduction

In application of the high temperature coatings such as CVD and TRD for metal-forming tools, substrates are quench- hardened during cool down from the coating temperature and/or during the post hardening stage.  The crystal phase transformation generated in such thermal cycles results in two types of distortion, dimensional change and deformation, as in ordinary hardening of steels.  The first distortion is a change in dimensions resulting from changes in the micro-structures of the substrate materials.  The latter is a change in shape, similar to bending in slender tools and “out-of-roundness” of cylindrical-shaped tools due to inhomogeneous micro-structures in the substrate materials and non- uniform heating and cooling during the coating process.

Bth dimensional change and deformation are more remarkably affected by the quality of substrate materials and by heat treatment rather than by coating. The coating operation, even if done under well-controlled conditions, cannot improve the distortion probability that is predetermined by the substrate materials and heat treatment method. Therefore, the selection of quality substrate materials is undoubtedly the first step to obtain surface coated tools with tight tolerances.

TRD coatings can produce carbide coatings on most metals, cermets, and ceramics containing carbon.  However, only limited types of materials can conform to the substrate materials of tools and wear parts used in material shaping operation such as metal stamping, forging, die casting, and cutting etc.  The materials to be used in tools with tight dimensional tolerances should be carefully considered.  This article briefly explains the potential problems related to substrate selection so that personnel responsible for the selection of tool materials can have a basic knowledge of how-to select good materials.

 

Requirements for tight distortion control

(A) Materials do not have any crystal transformation during TRD coating treatment.

The minimized distortion problem is realized in both dimension change and deformation, if the substrate materials do not have the volume change by the crystal phase transformation during the heating and cooling of the substrates.  Typical tool materials in this category are cemented carbides and cobalt alloys such as stellite.  Materials in this category are highly recommendable to some tools with extremely tight tolerances of  ± 10 µm or lese.

 

(B) Air hardening steels

Water quenching and oil quenching are likely to produce large deformation.   These steels produce more chances for dimensional changes due to scattering in depth of hardening.  Air hardening steels are highly recommendable. 

 

(C) Steels with high hardenability

If hardenability (ability for through hardening to core) of steels is not great and tools made of thick sections where only the surfaces of the substrate are fully hardened but the core is incompletely hardened, dimensional change of the substrates could be very inconsistent.     Steels with greater hardenability among many air-hardening steels are desirable for heavy sectioned tools.  For example, DC53 is better suited to heavy – sectioned tools than is D2.  The beginning in a drop in hardness at the center with increase in the steel diameter occurs at about 40 mm in D2 but 70mm in DC53. The decreased hardness at the core means imperfect hardening that is likely to produce inconsistent dimensional changes.

 

(D) Steels with less freedom in selection of hardening conditions

Tools subjected to TRD coating after being machined–, hardened–, finished and ground in most cases.  Differences in temperature, time and cooling rate between the preliminary hardening operation and TRD coating allow for big dimensional changes due to changes in the micro-structures, even if the substrates were fully hardened.  Nevertheless, information of the preliminary hardening condition from customers to TRD treaters is not usually done.  If the information of correct heat treatment is not supplied the use of the steels with a narrow range of hardening condition is highly recommended.  For example, D2 can be tempered either at the low range, 350 - 600°F or at the high range of 950 – 975 °F to get 57- 60 HRC.  This wide range, 1850 – 2150 °F for austenitizing and above 1000 °F for tempering is shown in a data sheet for CPM 15V.  On the other hand, DC53 is usually tempered at only high temperatures of 968 – 986 °F

   

(E) Steels can be hardened to their maximum hardness at the standard     TRD coating temperature

The steels being used with TRD coating most widely worldwide are D2 and H13 that have 1020-1040°C (1868–1904°F) of the recommended austenitizing temperatures.  This temperature range is selected as the standard TRD coating temperature by most TRD treaters.  Therefore, steels with the optimum austenitizing temperature similar to those of D2 and H13 can decrease the distortion problem due to the minimized differences of the condition between pre-hardening and TRD treatment.  Secondly, it will create the advantage of shorter delivery and reduced treating cost in comparison with the steels with different austenitizing temperatures.  Most steels can be loaded within the same charging in a TRD coating furnace, even if different treating times should be applied.

 

(F) No requirement for additional substrate hardening treatment

The highest coating temperature in TRD coating is lower than the optimum hardening temperatures for conventional wrought high speed steels and high tungsten hot working die steels.  In cases of high hardness such as HRC 63 or greater is needed for high speed steel substrates, the coated tools should be subjected to additional hardening treatment at 1150-1200°C (2100 – 2200°F).  This additional treatment can increase the likelihood of additional the distortion problems.  P/M high speed steels are strongly recommended in order to eliminate the need for the additional hardening treatment.   Hardness of HRC 62 - 63 or 65 – 67 can be easily obtained by the TRD coating at the standard temperatures- between 1025-1040°C (1875 –1900°F) for some types of P/M high speed steels. 

 

(G) Steels with capability of dimensional control by additional tempering keeping high hardness

Additional tempering at high temperatures such as 500-600ºC (930-1110ºF) on some types of steels can change dimensions due to change in microstructures of hardened substrates, without a large drop in hardness.  D-type steels such as D2, 8 % Cr cold working steels such as DC53 and Cruwear, and all types of high speed steels are well suited.

 

(H) Steels with greater reliability in quality

The degree of distortion can differ with both chemical composition and micro-structure even within the same steels.  There are large amounts of carbide particles in high carbon - highly alloyed steels such as D-type steels and high speed steels.  Amounts, size, shape, and segregation of the carbide particles have very large effect on the distortion problem. Distortion problems are highly affected by chemical composition of steels, and processing factors from casting into ingot to forging and rolling to the final steel stocks.  Thus the steels with steady quality is highly desirable.

 

(I) Steels produced by the same manufactures

It is wise to not use the AISI standard steels supplied by some or most steel makers in the USA.  They are introducing AISI standard steels from other steel makers in Asia and South America and selling it without notification.  Precise control of quality cannot be expected.
 

(J) Manufacturer’s brand steels rather than AISI standard steels

Steels with brand names, such as Vanadis by Bohler Uddeholm and DC53 by Daido are being exclusively produced by each steel maker themselves.  Therefore, using the brand steels means reduction in the variance in steel quality, which leads to a reduction in distortion problems. quality, lead to less distortion problem

 

(K) Powder metallurgical steels (P/M steels) rather than conventional wrought steels

P/M steels feature much smaller, uniform and much improved segregation of the carbide particles.  Reduced dispersion of dimension changes and smaller deformation such as “Out-of-roundness” can be expected.  

 

(L) Better materials rather than less expensive materials

Successful applications of TRD coating using the higher cost of better materials can produce much more profits when compared to TRD coating to the lower cost material and no application of TRD coating due to distortion problems.

 

Conclusion

There are many different types of tool materials in the world. Therefore, we are in a position where we can pick the best materials from the many types. Of course, we have to take many other properties into consideration, such as toughness, resistance to softening, hardenability, machinability, grindability, and price etc.  Wear resistance should not be considered since the tribological properties of the coated surfaces are independent from the substrate materials.  Similarly, resistance to wear and galling can be realized on a wide variety of materials from low alloyed steels to cemented carbides. Use of steels with very high content of vanadium, with more than 4% in wrought steels and 5% in P/M steels, is not recommended since higher vanadium content increases amounts of vanadium carbide particles in steels, resulting in poor toughness, poor machinability and grindability.  Extreme difficulty in grinding and polishing to achieve very smooth surface finishing on TRD coated tools likely to produce the “Out-of-roundness.

 

Table 1 shows the best materials for the tight distortion control that can be easily obtainable in the USA.  Selection was made based on the requirements discussed above.  Materials not recommended for tight control are also shown in Table 2 for easy cross  reference and explanation.

 

In both tables, the materials were classified with hardness that can be obtained normally by TRD coating with or without additional hardening treatment.  It is based on consideration of “Load carrying capability”

 

Regardless of type, thin hard coatings cannot work well, if the substrates plastically deform by mechanical loading, such as compression in which, strain of 1 % or more can be expected.  Therefore, the substrates of surface treated tools should have enough strength, “load carrying capability”, to eliminate such a deformation and Rockwell C Hardness (HRC) values can be used as a substitute for the load carrying capability measuring of which is time-consuming. 

 

The more severe the working condition the higher HRC is required.  However, it is very difficult to recognize how high HRC will be required for each application.  Extensive experiences in the applications of TRD coating shows that HRC values under normal condition may be 54-58 in metal stamping and 60-62 in cold forging of steels.  Under more severe conditions such as stamping and cold forging of heavy gauged metals, stainless steels, high strength steels etc., it should be higher by HRC 2-3.  

 

For normal hardness such as HRC 58-60, D2 is the best steel.  DC 53 is superior to D2 in almost all aspects, except difficulty for size control by additional tempering. DC53 is recommended for almost all application in which D2 and A2 are used, except in the case of very high tolerances as 10-20 µm. If “Out-of roundness” is the most important requirement, recommended materials are P/M die steels or P/M high speed steels for which the additional hardening is not needed to get the high hardness as HRC 62 or 65 or higher. 

 

There is no recommendation for good materials for tools used in hot forging in the tables.  Hot forging dies usually do not require such tight dimensional tolerances.  P/M high speed steels can be used with smaller distortion problems than conventional wrought hot working die steels and are recommended to the warm forging tools requiring tight tolerances.

 

Table 1 Good materials for tools requiring tight distortion control

 

Table 2 Not recommended materials for tools with tight distortion control

Remarks: All types of P/M high speed steels can be recommendable if they could be used without additional hardening and allowed to worse grindability and poor toughness 

P/M M3 or M4 type high speed steels: DEX 20, CPM REX M3, Micro-Melt M3, Vanadis 23, ASP 2023, CPM REX M4, Micro-Melt M4, ASP M4

 P/M with <3%V >8% Co: Micro-Melt M42, ASP 2017, DEX 40, CPM REX45, Micro-Melt HS30, Vanadis 30, ASP 2030, Micro-Melt HS76

(A) Materials do not have any crystal transformation during TRD coating treatment

(B) Air hardening steels

(C) Steels with high hardenability

(D) Steels with less freedom in selection of hardening conditions

(E) Steels can be hardened to their maximum hardness at the standard TRD coating temperature

(F) No requirement for additional substrate hardening treatment

(G) Steels with capability of dimensional control by additional tempering

(H) Steels with greater reliability in quality

(I) Steels produced by the same manufactures

(J) Manufacturer’s brand steels rather than AISI standard steels

(K) Powder metallurgical steels (P/M steels) rather than conventional wrought steels

(L) Better materials rather than less expensive materials

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