Usually, conventional welding is a ‘Repair-Oriented’ technique. It is carried out after the dies are damaged or worn out. A new Japanese cold-welding technique enables appropriate surface hardening of dies, moulds and tools to increase their service life, says Srikar Shenoy.

Die, mould and tool wear are major reasons for production downtime and increased costs in most industries. Apart from using strong base metals for making dies, a few effective treatments can be administered to dies to increase their service life. Even if possible, such treatments are not feasible for all metal-forming units. This article introduces a practical and economical technique to increase die, mould and tool life. The technique can be easily adopted by all metal-forming units, big and small. Nitriding and PVD, CVD are most popular treatments to increase die life.

Nitriding: Gas nitriding/plasma ion nitriding are popular surface hardening treatments carried out on dies. With an initial die hardness of 42 HRC, the nitriding process can further harden the surface of die up to 64 HRC. Many large forge shops in India carry out nitriding of 100% of their forging dies. However, very few forging companies have an in-house nitriding facility. Nitriding is found effective in many cases. The skill of the nitrider, flawless nitriding facility and process play a vital role in the success of this technique. Nitriding is also a capital-intensive technique and not many companies can afford the finance, space and skilled workforce to set up an in-house nitriding facility. Getting the dies nitrided from commercial heat treat shops is not always feasible. Finally, additional efforts are required to carry out selective nitriding and mask areas of components where nitriding is to be avoided.

PVD, CVD: Treatments like PVD (Physical Vapour Deposition) and CVD (Chemical Vapour Deposition) are popular in the machine-tool industries in America and European countries. These techniques are yet to evolve completely in the Indian forging industry. PVD is slowly gaining share in the Indian machine-tool treatment market. It ensures surface hardness of up to 90 HRC. However, the result of PVD treatment on forging dies is not documented. Cost of PVD is approximately Rs 800 per kg of metal that is treated (as on date of this article). CVD technology is not yet introduced in India.

Welding to repair/reclaim worn out dies
Apart from the mentioned treatments, the only option available is to repair and reclaim the dies through welding. Conventionally, welding of the worn out areas of dies or welding of cracks in dies is carried out. Flood welding of the dies is also carried out to completely reclaim the dies. However, this technique also poses its own limitations.
Limitations of conventional welding:

•  Requirement of skilled welders
•  Requires open space or effective ducting for carrying out welding operation
•  Time consuming process as forging dies must be taken to welding area
•  Pre and post welding heat treatment / stress relieving is necessary

Usually, conventional welding is a ‘Repair-Oriented’ technique. It is carried out after the dies are damaged or worn out.

Japanese cold welding: New protective treatment to increase die life
A new Japanese cold-welding technique enables appropriate surface hardening of dies, moulds and tools to increase their service life. The technique involves electronic coating of tungsten carbide on selective wear-prone areas of dies/moulds/tools through Japanese cold welding technique.

Cold welding is carried out as a ‘Preventive Maintenance’ technique on new dies. It is a surface hardening technique, similar to nitriding and PVD, but is administered using a completely different methodology. Hardness of tungsten carbide layer deposited by cold-welding on dies can surpass nitriding to reach hardness of more than 70 HRC. Figure 1 show photographs indicating beginning of die wear on forging dies are given below.

Benefits of Japanese cold welding technology are:

• It does not need skilled welders and can be carried out by anyone
• There is no need of open space. No fumes are generated during cold welding
• It is a time saving process as dies need not be removed from forging equipment

Pre and post welding heat treatment is not necessary. No stresses are generated during cold welding as it is a cold process
Additional benefits of cold welding technology are:

• Nitriding of dies not required as hardness of tungsten carbide coating is more than 70 HRC, which is higher than nitriding hardness (62-64 HRC).
•  Increased die life due to high wear resistance
• Substantially reduced maintenance downtime of dies and tools
•  It can be carried out on selective areas of dies that are prone to wear. Does not require the complete die to be treated/ protected Argon gas cylinder is shown in Figure 2. It is recommended to be used during the cold welding process to avoid oxidation of carbide electrode. Compact cold welding process can be used by anyone, anywhere. Figure 3 shows some of the applications of Compact cold welding process.

Principle of operation
Consumable electrode made of alloys and intermetallic compounds are deposited on the die by means of electronic spark at a frequency f 10?¹ to 10?³ second for one to millionth of seconds per spark. Direct current from the power supply will heat the electrode to 8,000°C to 25,000°C, only at the contact areas and transfer a small quantity of electrode to the work piece under an ionized state. A strong metallurgical bond is thereby produced.

Characteristics of carbide coating are:
1.    Wear resistant: Due to inherent strength of tungsten carbide, the wear resistance is high. If the die is hard and heat treated well, a good forging die life can be expected after carbide protective coating.
2.    Heat resistant: The coating is heat resistant and will not cause heat checks. Excessive heat leading to die wear will be prevented in protected areas.
3.    Scuffing resistant: Scuffing and bruising is the initial stage of having serrations on die. This scuffing will be prevented or substantially delayed.
4.    Lubricity: Many times, due to very smooth finish of new dies, the forging / casting die lubricant does not adhere to the die. This problem is not faced in the case of carbide coated dies. It is observed that die can be lubricated better than before.
Table 1 shows comparison of cold welding with other techniques.

Successful cold-welding case studies
A number of esteemed Indian metal-forming organisations have tried out carbide coating on wear-prone areas of their dies and tools using Japanese cold-welding technique. Figure 4 shows photographs of some of the products made using cold-welding.
Table 2 shows that substantial increase in die life after cold-welding.

Summary
Japanese cold welding technique of electronically overlaying protective carbide layer on metal-forming dies and tools holds promises of increased die & tool life, reduced maintenance downtime, convenience of operation and better productivity. As the concept is proven, it is a requirement in every modern metal-forming unit seeking cost reduction and increased profitability.

About the Author:
Srikar Shenoy is the Director of Steel Plant Specialities LLP. For details, contact on Tel: +91 22 6797 8060, 25552459; Mob: +91 98204 93373; E-mail: info@steelplantspecialities.com, or visit
Website: www.steelplantspecialities.com