Facing extensive wear of punch coatings often happens due to increasingly high temperatures. There are many options available for punch coatings but only a few are for high-tensile stamping, offering unique features and extreme flexibility. The MISUMI Die Mold product engineering team performed a few tests to compare the coatings and in this post, we review the results.
RW Coating
RW Coating is a composite coating that undergoes the WPC® layer treatment at the base of the coating and is lined with a layer of Al-Cr (Aluminum-Chromium) type coating on the top.
It has a hardness degree as high as 3100 HV (Vickers Hardness).
Since it is much harder than its predecessor, it is ideal for applications in which premature wear and tear of the punch cutting edge occur.
HW Coating
HW Coating is a composite coating that also undergoes the WPC® layer treatment which increases surface hardness and toughness of the base material SKH51 and improves adhesion with TiCN.
Rockwell Impression Test
The RW coating and HW coating were evaluated for the adhesive capability of the film through a Rockwell Impression test.
Test Conditions:
Test load: 150kgf
Punch base material: Powdered high-speed steel.
As seen in the test image above, RW coating is able to withstand the deformation of the base layer of the punch and no cracks were observed. It is a coating that achieves a balance between both the hardness and the improved adhesive capability of the film. For reference, in HW coating, the TiCN film could not withstand the bulging in the base layer of the punch, and numerous cracks appeared around the Rockwell impression.
Heat and Temperature Resistance Tests
Test conditions:
Temperature: 1000 °C
Time: 540 Minutes
Heat and temperature resistance tests were conducted for the steel that was subjected to RW and HW coating, and the structural observation of the film’s cross-section was done.
We can see that when HW coating was held at 1000 degrees C for 5 min, the Ti of the film was changed to coarse oxide. Contrarily, when the RW coating was held at 1000 degrees C for 540 min, it retained its fine structure, and thus, we were able to confirm that it has superior heat resistance. The heat resistance (oxidation temperature) of the RW coating and other various kinds of coatings has been summarized in the graph below.
In the rigorous pressing treatment of high tensile strength steel sheets or thick plate high tensile strength sheets, the temperature of the cutting edge of the punch will rise steadily.
When faced with premature wearing off of the coating, often caused by the increased temperature of the punch cutting edge causes the coating to peel off.
Punching Test
Punching tests were performed for each coating to see which coating lasted the longest.
Punching Conditions
Processed material: 590 MPa class high tensile strength steel sheet
Sheet thickness: 10mm (0.9mm coining surface)
Punching speed: 35 spm
Lubrication: Oil and chlorine free type coolant
Punching shape: ɸ10.45mm
Cutting edge stress: 223kgf/mm²
Clearance: 7%
Punch: Tapped head punch RW coating
In the HW coating, peeling began to develop at 50,000 shots, and the test had to be suspended, but RW coating, only some minor wearing of the coating was observed even at 100,000 shots. Also, we observed RW coating still very much in usable condition.
For the competitor’s Al-Cr type coating, peeling was restricted to some areas at 50,000 shots, but at 70,000 shots peeling progressed rapidly and the wear and tear of the punch base material were observed.
Bonus Coating – RX
The RX coating is a composite coating that undergoes a lapping finish and special nitriding treatment at the base material and is lined with a layer of Al-Cr type coating on the surface. Lapping finish and special nitriding treatment improve the adhesive performance of the coating and prevent the coating film from peeling off. Due to the excellent surface roughness of the punch, it improves the sliding property against the work material and prevents galling.
Coating Map
(Assuming A Steel Workpiece)
MISUMI’s new punch coating options are designed with superior resistance to high temperatures, saving you time and money. You can learn more about our high-tensile stamping punch coating options to determine which is best for you. To explore more, visit our specialty punch coatings page here.
