Application Techniques of TD Process Mainly to Large Sheet Metal Stamping Dies

1.      Foreword
The application of TD process to sheet metal stamping dies provides a significant industrial merit caused by the improved performance of the dies.  Therefore, TD Process is practically employed in the wide range of stamping field in several countries including Japan.

However, a wrong application of TD Process does not bring the advantage.  This article briefly summarizes some instructions on applying TD Process that are desired to be known by the users.  We hope these instructions become widely prevalent among users.  This article contains some common items about dies for application other than sheet metal stamping, and in addition, items about various application objects other than dies.

2.      Stamping dies suitable for application of TD Process
The carbide layer formed by TD Process is excellent in seizure resistance (scoring resistance), and wear resistance.  Therefore, TD Process is suitable for those stamping dies that are liable to be suffered from scoring and wear causing the following problems:

1. The dies have to be frequently repaired and replaced.
2. High graded lubricants or a great amount of lubricant oil have to be used.
3. Phosphate coating or PVC coating must be applied to the blank material.
4. A great deal of labor is required for removing scoring flaws of the product.
5. The number of stamping process steps must be increased.

Those dies as described above include the dies to form hard-to-work materials such as high strength steel, plated steel, stainless steel and aluminum.  Those dies that make a great number of stamping even when the material is easy to work like mild steel are also included.

In other words, these dies are always exposed to the demand of further improvement even when high class materials such as cold working die steel and high speed steel are used, or when surface treatments such as chromium plating and nitriding are applied to these materials.

The TD Process can provide the substrate with almost the same hardness as that of usual hardened steel dies with little loss of toughness.  Accordingly, the process should be applicable with no particular problems to the steel substrate in the application where hardened steel dies were used.  Also the process can be applied to dies to which non-ferrous metals such as copper alloys and zinc alloys or cast iron were conventionally used by altering the substrate from such alloys to steel.  The use of cast iron as the substrate also will be possible under less severe working conditions.

Though the  application of TD Process to cemented carbide dies is possible in many cases, the actual application should be very careful in the case where damages to cemented carbide dies are frequently encountered because the n phase which is liable to be produced by TD Process can reduce toughness of dies and damages may readily occur.  In such cases, those cemented carbides and the TD Process conditions that are hard to produce n phase must be selected.  When cemented carbide dies are used because of their wear resistance and scoring resistance rather their strength dies of characteristics superior to those of cemented carbide dies can be made by applying TD Process to steel substrate.

Though the effect of TD Process is affected by the work material, there is no work material to which TD Process is not applicable.  The effect of employment of TD Process is affected by also the surface condition of work material.  For steel, the effect on the as hot rolled sheet and the shot blasted sheet is smaller than that on descaled sheet by pickling.

3.      Die design
3.1 Division of large die
The die used for forming of a large part such as an automobile member is made by assembling small divided die pieces.  Such divisions of the die should observe in the Fig. 1.

The division should be avoided at such places that the configuration changes steeply, and scoring or wear is likely to occur; i.e., beads, die curvatures and shrinkage flanges.  Die pieces on which damages are liable to occur, and frequent repair and re-TD-treatments are required to be as small as possible.  It provides the ease of repair and reprocessing operation, resulting in cost reduction.  The method of division should be selected so that as little distortion as possible occurs on each divided piece of the die when subjected to TD Process.  A configuration hardly causing distortion is the one as simple as possible and with no significant difference in section thickness at any place (Fig. 2).

The die division line should be perpendicular to the contour line as far as possible (Fig. 3).  Troubles including scoring and damaged layer are liable to occur on edges made with acute angles.  Edges of division lines are to be rounded with a radius of 0.2 – 0.6 mm (Fig.4).  Otherwise, scoring and damage to the coating layer may be caused even by a slight unevenness of level which occurs in assembling the die.  There is little possibility of trouble occurrence due to penetration of work material into the rounded rooms.

A slight distortion may occur on each die piece by applying TD Process requiring grinding on their joint surfaces or back surfaces (assembled side) for dimensional adjustment when the pieces are assembled into the complete die.  To facilitate the grinding, it is recommended that relieves as shown in Fig. 5 be provided in advance.

3.2   Dimensions
The substrate has to be hardened simultaneously with TD Process or by reheating (mostly for high speed steel substrate).  If the substrate has not been hardened before applying TD Process, the microstructure of the substrate materials turns from pearlite (annealed state) to martensite plus retained austenite (hardened state) leading to a large dimensional change of the substrate material.  The amount of this dimensional change is very hard to predict because the change is governed largely by many factors.

Examples of dimensional change of dies hardened under standard conditions for the type of the steel when subjected to TD Process (accompanied by hardening of the substrate) under standard conditions are as shown in Fig. 6 -11 for reference.  These graphs were prepared on articles to which TD Process was applied by the commercial processors without sufficient prior discussion between the TD Processors and the user.  Although plotted changes in diameter scatter to some extend, the dimensional change is as narrow as 10-30 u.m.  These values are smaller in comparison with the case of ordinary hardening treatment.

The change and its dispersion are caused by differences and their dispersion of initial hardening conditions and TD Process conditions besides other factors.  The change and its dispersion can be reduced by reducing the differences.  This essential particularly when relatively small dimensional tolerances are assigned.  To minimize differences of conditions, a sufficient talk will be necessary between the user and the TD processors.

Unless the dimensional tolerance is much larger than the amount of change, such a dimensional change must be added to or subtracted from the final dimension (see 5.2 when the tolerance is as small as + or – 5 um).

The followings are examples of dimensional requirements for dies before subjected to TD Process presented by the processors to the user.

AISI D2 preliminarily hardened at 1000 – 1050 °C (austenitizing) and 150 – 200 °C (tempering).

Punch (5 – 10 mm dia.): - 0.05% (-0.01) of low limit of finishing dimension

Ring die: = 0.01 of medium value of finishing dimension.

For the case, like precision perforating punches, where severe tolerances are required in warpage and shank diameter, the shank is recommended to be finished by grinding after TD Process as shown in Fig. 12, and a grinding allowance is to be left to secure the warpage removal and the accurate diameter.  The necessary grinding allowance will be 0.l – 0.5 mm though it depends on the configuration and dimensions of die and TD Process condition, etc.

3.3  Configuration
In the case of drawing dies, scoring and wear are hardly to occur as the drawing radius R increases.  Accordingly, R should be as large as possible so long as the designing situation permits.  For dies used in deformed punching, the largest radius R allowable from the standpoint of the external view of the product should be given to edges.  The radius R should be given to as many edges as possible to prevent cracking when in preliminary hardening, TD processing and when in use (Fig. 13).  To minimize warpage, it is desirable to make a change in configuration for symmetrical design when possible as shown in Fig. 14.

3.4  Positions of holes
To prevent quenching cracks in preliminary hardening and TD processing, holes of die should not be too near the peripheral edge (Fig. 15).  For those dies liable to cause a warpage and to be assembled with bolt tightening, the number of the bolt holes should be increased (Fig. 16).  Mounting holes should be arranged symmetrically as far as the situation permits (Fig. 17).

4.      Substrate Material Selection
For the die to be subjected to TD Process, the user of air hardening die steels which can be quenched by slow cooling such as D2 is desirable.  Low hardenability die steel such as D3 which require oil quenching are liable to cause a quenching distortion, and hardening scarcely reaches the core leading to the occurrence of dimensional change.  (When preliminary hardening is employed, hardening reaches the core in many cases since the die may be cooled at higher cooling rate in comparison with the quenching in TD Process, even when quenched in oil).

Shallow hardenability steels such as carbon tool steel, low alloyed tool steel and cast iron can be used when the dimensional allowance is relatively large.  Air hardening low alloyed tool steels with improved hardenability are recommended for dies of relatively severe dimensional allowance.

For the dies loaded by a large pressure (e.g., 200kg/mm2 or above) used for forming thick plates, and forming of sand blasted steel sheet, high speed steel that can provide a large substrate hardness is recommendable.

Ordinary wrought high speed steel requires reheating for quenching since a substrate hardness exceeding HRC60 is not obtained in the TD Process at normal temperature.  Powder high speed steel is recommended, despite its high price, because of its low level of distortion and a hardness of HRC 64 - 70 obtained merely in the TD Process.

5.      Dies Manufacturing
5.1  Rough machining

Taking into account the rolling direction, performed material cutting so as to minimize the distortion (Figs. 18 and 19).  The curve of radius R should be smoothly jointed to straight line (Fig. 20).

5.2  Preliminary heat treatment
The dies with severe dimensional precision are to be hardened before the finish grinding under conditions recommended for the steel in JIS, AISI or other standards.  Selected hardening conditions should be informed in details to the TD processor who can select the TD Process conditions to cause the minimum possible distortion.

For punches with relatively small dimensions, simple configuration, and severe dimensional allowance, e.g., standard punches, it is desirable to  perform tempering at a temperature which allows complete decomposition of retained austenite, and leads to the minimum possible hardness drop (e.g., 520 – 540 °C for D2).  This is called as the high temperature tempering in contrast with the low temperature tempering usually performed at 150-200 °C.  The same high tempering temperature is desirable after the application of TD Process because the dimensional change of the substrate due to the difference in austenite quantity before and after TD processing is forced to a minimum level by selecting such a tempering temperature.  Fig. 21 shows the effect of high temperature tempering on the dispersion of measured values of the punch diameter.  Fig. 22 shows the diameter dispersion plotted from 1700 pieces of punch most of which are converged within a range of + or - 5 mm.  A great number of standard punches that have a tip diameter allowance of + or – 5 mm and shank diameter of 3 – 25 mm are commercially available in Japan (Fig. 23).

Instead of the high temperature tempering, the method in which the retained austenite is decomposed by sub-zero treatment is available for the same purpose.  Since this method hardly eliminates the retained austenite completely, the high temperature tempering is practically desirable.

5.3  Finishing
Key points to be borne in mind are the surface roughness and polishing direction.  As the surface layer produced by TD Process is very hard, the rough surface of the die makes flaws on the work material causing a negative effect of TD Process application.  The allowable limit of the surface roughness depends largely on the type of forming, reduction in thickness, and the type of the work material, but the die surface should be finished to at least Rmax 3 mm or less.  When plated steel sheet, stainless steel sheet, high strength steel and aluminum sheet are worked, finishing of Rmax 1 mm or desirably 0.5 mm is recommended.

This polishing is performed mostly by buffing after grinding with emery papers starting from #200 and successively to a finer paper of #600, then #1000 and so on.   It is recommended that the polishing mark is in parallel with the metal flow (Fig. 24).

Polishing is necessary of course for only those surfaces that contact the work material.  Particularly, those area in which scoring and wear are liable to occur should be more smoothly finished then other areas.

When the die is subjected to electrical discharge machining, an abnormal layer consisting of the melted and solidified layer, hardened layer and tempered layer is formed from the surface toward the interior. The melted and solidified layer contains cracks and pores, and the chemical composition of the layer is affected by contamination due to the electrode and machining fluid.  Sufficient improvement of the die performance can not be expected by application of TD Process if this fouled layer is left intact.  Consequently, the layer should be removed wholly, or at least to a depth where cracks and pores are no longer recognized by grinding before subjected to TD Process.  Hardened and tempered layer exert nearly no detrimental effect because they are charged to normal structure when subjected to TD Process.

Burrs that generated by grinding at cutting edges of shear blades and blanking dies must be completely removed in advance.  Otherwise, no improvement of die performance can be expected even after applying TD Process.

Burrs may be left if the die is used with its front surface ground because the remaining burrs are removed by grinding.

5.4  Measures for existing surface treatment
5.4.1        Miscellaneous Surface Treatments
When TD Process is to be applied to the dies to which nitriding or low temperature carbonitriding was applied, it is acceptable to remove the surface compound layer by grinding, but the removal is not absolutely necessary.  When the process is applied without removing the compound layer, a carbonitride layer whose characteristics are similar to those of the carbide layer will be formed to exhibit the almost equivalent practical performance.

When the die has a chromium plating on its surface, the die may be subjected to TD Process without removing the plating layer if its thickness is below 30 um.  In this case, the intended carbide layer is formed on the Cr-7C3 layer originated from the plating layer with no particular problem in interlayer bonding strength except that the plating layer is desired to be removed when the plating thickness exceeds 30 mm and even when the thickness is below 30 mm.

When the die has a nickel plating layer, a soft plating layer remains between the carbide layer and substrate material after the application of TD Process.  The nickel plating layer should be removed in advance because such a soft layer can not withstand the use under a high surface pressure.

Those coating layer of TiC and TiN by CVD, and Tin by PVD also should be removed before applying TD Process.  Otherwise, the carbide or carbonitride layer is formed on such a coating layer to cause a poor bonding strength particularly between TiN and  the substrate material leading to a trouble.

TD Process can not be applied to build-up beads of copper alloy.

5.4.2.      TD Process (Re-treatment)
When the working surface of a drawing die or a bending die with TD Process has been partly damaged during the use.  The damage surface is to be repaired by polishing with a grinder, oil stone, emery paper on the buff till the damage is removed and the necessary surface smoothness is obtained.  At the same time, the repaired surface should be smoothly connected to surfaces that are not repaired. It is quite unnecessary to remove the carbide layer over the whole working surface of the die because the growth rate of the carbide layer is controlled by diffusion (the layer thickness is proportional to the square root of time), and a thick carbide layer is formed only on the surface not coated with carbide layer, and thus no stepwise unevenness is resulted.  The carbide layer is formed also on the previously existing layer, but there is no fear of exfoliation on the boundary between new and old layers.  This is because the TD Process consists of the salt bath method using borax as the major reagent.  This advantage is not expected with CVD and PVD.

TD Process can be applied repeatedly to dies.  The maximum allowable number of repetition depends on working conditions, configuration, dimensions and substrate material of the die as well as conditions of TD Process.  Repetition of several times may be possible though a definite number can not be shown.  A repetition exceeding eight times is reported in Japan.

6.      TD Process Treatment
To secure a sufficient substrate hardness with minimum distortion, the die is to be set to the rack of an appropriate configuration and dimensions with appropriate position and distance between each dies (when plural dies are simultaneously treated).   After as many steps of preheating as possible, the die is loaded into the TD furnace regulated at a temperature specified mainly according to the type of substrate steel, and cooled by an appropriate means after holding the die for the specified period of time at the furnace temperature.  The holding time is designated on the basis of the time necessary for obtaining the desired carbide layer thickness, but the time is affected also by the furnace temperature and the type of substrate material.  Generally, the time is 1 – 10 hours.  For shear blades and blanking dies, a carbide thickness of two to several um is selected, and a thickness of several to as large as ten u.m. is selected for other dies.

The dies after cooled are subjected to tempering and cleaning for salt removal.  Temperature of TD Process and tempering are desired to be nearly similar to the temperature of the preliminary hardening.  A high temperature tempering is preferable for dies that are prone to be broken.

Important key points in the practice of TD Process are to provide the necessary substrate hardness and to minimize the distortion.  Both are significantly affected by the cooling condition of the die after removal from the TD Process furnace.  Fig. 25 shows the effect of cooling rate on the amount of distortion in standard punch manufacturing described before indicating that air cooling is very effective for preventing the warpage.

7.      Mounting of die to press
As shown in Fig. 26, no change in surface roughness due to TD Process is seen in the range of roughness above 1 um.  In the region below 1 um, however, the roughness increases up to about 1um by applying TD Process.  Therefore, when a roughness below 1 um is desired, the necessary lapping should be made with a small consumption of diamond paste (e.g.,3 um particle size) after the application of TD Process.  Other hard abrasive grain can be used with an additional time.

When the divided die segments are assembled, the back surface and joint surfaces are to be ground or interposing shims should be employed in order to avoid occurrence of stepwise unevenness at working surfaces.

When making for preventing carbide layer formation is not performed on the die before applying TD Process, working is facilitated by removing the carbide layer through sand blasting before grinding.  Even when the carbide layer is present, grinding with ordinary GC and WA grinding stones can be done without difficulties if the grinding is carried out with deep cuttings which reach the substrate material. Grinding with shallower cuttings is possible if the corner carbide is removed first as shown in Fig. 27.  If working surfaces are ground, the carbide layer is removed unwontedly and the purpose of TD Process is not accomplished.  Despite the above clear fact, this mistake is often committed in the production plant.

Cutting edges of shear blades and punching dies are often bruised by improper handling in transit.  Dies are subjected to TD Process occasionally with burrs on the cutting edge.  Such dies should be used after making the cutting edges free from faults by grinding the front surface.  This removal of the front carbide layer is regardless of the die performance because the cutting performance is governed merely by the cutting edge consisting of the carbide layer on the side surface.  Thus, the grinding can be repeated without any performance drop.  Many actual examples of application show that the use of die without knowing the presence of burrs and bruise, and thus without grinding causes a poor result till a satisfactory result is obtained after grinding.

The type of the grinding stone, and amounts of cutting pitch and feed should be carefully selected to avoid chipping on the carbide layer during the grinding though these conditions are a little less strict than those in ordinary grinding. The grinding is easily practiced with ordinary alundum stone.

8.      Stamping Works
Both wear resistance and scoring resistance of dies are considerably improved by applying TD Process.  The improvement can reduce the lubricant consumption, and replace the lubricant by one more inexpensive and of easier after treatment in many cases.  Higher working rate and man-hour reduction in quality control inspections are also possible.  Efforts should be made without being satisfied merely by increase in service life and the profit originated from reduction of repairing man-hours, but with the positive intention of obtaining potential profits.  Reduction of lubricant consumption, for example, is easily realized by reducing the usage of lubricant by small amounts watching the result each time.

9.      Repairing By Built-Up Welding
Besides damages to a large extent by the use of dies, there are many cases where built-up welding has to be made for the reason of design changes and design failures.  To perform repairing by built-up welding on TD processed dies, the procedure same as that used for dies not subjected to TD Process is to be applied.

When the application of TD Process after repairing by built-up welding is desired, the optimum build-up material should be selected.  The build-up of weld beads of the same chemical composition as the substrate material is ideal for the purpose.  Requirements for weld beads are:

1. Analogousness of transformation behavior in heating and cooling to the substrate material
2. Quenched hardness obtained from TD Process almost the same as that of the substrate material
3. The same growth rate of carbide layer as that on the substrate material

Another requirement of the weld bead material is the ease of welding operation.  It is hard for any materials coated electrodes 85 (Tokushu Denkyoku K.K.) etc. are used for the substrate of cold working die steel. Austenitic weld bead materials are not acceptable for the substrate of cold working die steel in all requirements of (1) through (3).

When a die repaired by built-up welding is subjected to TD Process, never fail to notify to the TD Processor of the fact that the die was repaired by build-up welding.

10.  Conclusion
Instructions for steps before and after applying TD Process to sheet metal stamping dies, and some related points to be dealt with at the user were outlined.  There are various manners to cope with problems in the same situation, and the background for contrivance in the practice.  Cooperation with the TD Processor is effective for solving the problem with more ease and efficiency.

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