Protective coatings continue to play a major role in increasing productivity and reducing costs in metal forming and treatment processes like hot forging and heat treatment.

With recent advancements, nanotechnology has been introduced in the manufacturing of protective coatings for metal forming processes. This technical paper presents details and successful case studies of three such protective coatings: 

1. Die, mould and tool wear are major reasons for production downtime and increased costs in most industries. Apart from using suitable alloy steels for making dies, a few effective treatments like nitriding, PVD and CVD can be administered to increase die service life. Even if possible, such treatments are not feasible for all metal forming units. Carbide coating to protect only the wear-prone areas of dies using Japanese cold-welding technology is a practical and economical technique that has proven to increase die, mould and tool life. This technique, though similar to welding, does not pose difficulties of smoke emission, pre and post-weld heat treatment and requirement of skilled labour. It can also be carried out on the die or tool without unloading it from the forging press or such equipment, without need of a weld shop.

2. When forging die is in use, it is mandatory to keep it well lubricated and the die temperature maintained as per required application. Die protective coating cum lubricant is used to achieve these objectives. Graphite-in-water formulations are popularly used as die lubricants until recently. Though effective as a lubricant, graphite has proven to be highly polluting and dirties the surroundings. Effective white lubricants using environment friendly materials are now developed that have proven to eliminate graphite and associated pollution. Substantial increase in die life and reduced pollution is possible by the use of white lubricant cum protective die coating. 

3. Oxidation and resultant scaling at high temperatures is caused during heating of billets, ingots for forging and during heat treatment of formed components. Scaling leads to enormous losses by way of rejections of produce, reduced yield and increase in non-value adding operations like shot-blasting, grinding, pickling, etc. These parameters are becoming increasingly sensitive in open and closed die forging, especially of expensive grades of steel like SS, Nickel-bearing steels and aerospace forgings. Anti-scale protective coatings can be used to prevent or substantially reduce high-temperature oxidation and scaling. Nano-material based protective coating have proven to reduce rejections, reduce quench cracks, improve surface finish of parts, reduce shot blasting and increase yield. This technique is also proven useful in protecting dies and tools during heat treatment.

All these techniques can be easily adopted by all metal forming units, big and small. 

Subject 1: Increasing forging die and tool life by the use of Japanese cold-welding technique

Problem: 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 forging units, e.g., purchasing and maintaining an in-house nitriding facility is not feasible for most forging units.

Observation: Most forging dies wear out only in certain areas. The whole die block does not wear out at once. Only sensitive portions of the die, like edges, profiles that take majority forging load, etc. wear out much faster than the rest of the die profile. Some examples are shown below:

Technology: Japanese cold-welding technique enables appropriate surface hardening of dies, moulds and tools to increase their service life. The technique involves coating of tungsten carbide on selective wear-prone areas of dies/ moulds / tools through the special electronic 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 manually using the Japanese cold-welding equipment. Hardness of tungsten carbide layer deposited by cold-welding on dies can surpass nitriding to reach hardness of more than 70 HRC.

Benefits of Japanese cold welding technology:

1. Skilled welders not required – can be carried out by anyone

2. Open space not required – no fumes are generated during cold welding

3. Time saving process as dies need not be removed from forging equipment, and

4. Pre and post welding heat treatment not necessary – no stresses are generated during cold welding as it is a cold process.

Additional benefits of Japanese cold welding technology:

1. 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)

2. Increased die life due to high wear resistance

3. Substantially reduced maintenance downtime of dies and tools, and

4. Can be carried out on selective areas of dies that are prone to wear – does not require the complete die to be treated/ protected.

Compact cold welding equipment and applicators

Can be used by anyone, anywhere

Carbide coating is seen as silvery, coarse coating on wear-prone areas of dies

Substantial increase in die life 

after cold-welding

Conclusion 1: Till date, no negative result is observed in any of the demonstrations of this technique carried out in various metal forming operations. Hence, there is absolutely no risk in terms of die/tool breakage or reduced life. The percentage of increase in die and tool life has varied from as low as 14% in initial trials to as high as 120% in latest trials. Various parameters that contribute to success of this technique are well documented, leading to refinement of the technique. This has assured better results on subsequent demonstrations on hot forging dies.

Subject 2: Increasing die life using customised die lubricating equipment and environment friendly die lubricants 

Problem: Die lubricants play an important role in achieving optimum die life. Cheap oils when used as die lubricant, are functional, however offer very low die life and pollute the forge shop. Water-based graphite are better than oils, however, they too dirty the forge shop.

Observation: Many times, switching over from oil to water-based graphite or to synthetic white lubricants is daunting. This is due to improper spray techniques, leading to low die life or die breakage.

Technology: New generation synthetic die lubricants that are effective and clean are now available. Synthetic die lubricants have often proven better compared with graphite. This is possible only when synthetic lubricants are used with correct method of spraying. Customised spraying systems and spray guns, depending on the specific forging profile need to be used.

Comparison Case Study I: Productivity Improvement due to reduced die grinding time by switching over to graphiteless, water soluble lubricant.

Comparison Case Study II: Cost Saving due to graphiteless, water soluble forging lubricant.

Conclusion 2: Substantial increase in die life is possible by the use of environment friendly die lubricants when correct spraying equipment and spraying techniques are implemented.

Subject 3: Reducing rejections and increasing yield using anti-scale protective coatings

Problem: Oxidation and resultant scaling leads to pit-marks and rejections. Non-value adding operations like shot blasting, grinding, etc., are costly and time consuming.

Observation: Oxidation and scaling are a function of time, temperature and the thermodynamic affinity between oxygen and metal. Recent developments in highly oxidation-prone grades like nickel bearing steels, high speed steels and stringent customer demands do not allow for scale pits, uncontrolled decarburisation and bad surface finish.

Technology: An anti-scale coating is applied on billets or components to be heated before charging them into furnace. This anti-scale coating acts as a barrier between oxygen and metal. Care is taken to apply a uniform, impervious layer of coating by brushing, spraying or dipping. 

Use of anti-scale protective coatings on billets during heating for forging and again on forgings during heat treatment have proven to substantially reduce scaling, control decarburisation, improve surface finish and increase yield.

hotos below show substantially reduced scaling on billets. As a result, forged parts do not have scale-pits and have acceptable surface finish.

Conclusion 3: Benefits proven by the use of anti-scale protective coatings are:

Substantially reduced scaling – reduced rejections due to scale pits.

Shot blasting/acid pickling time is either reduced to a great extent or eliminated.

Consistently controlled decarburisation.

Increased yield. 

Subject 4: Reducing heat treatment defects like cracks, distortions and warping by accurate non-destructive measurement of microstructure, surface to core cooling rate, quenchant health and agitation uniformity

Problem: Drastic variations in microstructure and hardness of heat treated parts lead to rejections due to cracking, warping and distortions. At present, microstructure analysis is carried out by destructive procedures, after heat treatment. It is not possible to pre-determine the results that will be obtained after heat treatment. 

Observation: Due to developments in quenchants, forge shops are switching over from quenching oils to safe polymer quenchants. Challenges like quench cracks, distortions, etc. are faced during change-over. In all types of quenching operations, it is desirable to know the results of heat treatment before carrying out the actual batch heat treatment. 

Technology: QuenchProbe can be used for estimating the hardness and microstructure variations during quenching of an alloy steel specimen, in plant conditions. The health of the quenchant also can be monitored directly with Reference QuenchProbe as the performance of any quenchant deteriorates with time and usage. A combination of hardware (probe) and the accompanying software have proven to achieve manifold benefits.

Immersion quench heat treatment of steel refers to the process of heating steel component to a suitable temperature and immersing it in a fluid medium. The fluid medium is called the quenchant. Quenchants can be of many types; brine, various types of mineral oils, aqueous solutions of polymers, or even plain water. 

When a steel component is cooled from the ‘austenetizing temperature’ (around 850oC for most of the steels) the austenite transforms itself into various phases depending upon the cooling rate. If the cooling is very fast, like in quenching, more of martensite and bainite will form making the component hard. The percentages of these phases in the heat treated component control the hardness. QuenchProbe can be used for estimating the hardness and microstructure variations during quenching of an alloy steel specimen, in plant conditions. The health of the quenchant also can be monitored directly with Reference QuenchProbe as the performance of any quenchant deteriorates with time and usage.

Unique features of Reference Quenchprobe and our advanced SOFTWARE 

1.    Cylindrical specimen made of same grade as the component (20 mm to 50 mm diameter)  

2.    Tests done at actual quenching condition

3.    USB based data logger for data portability 

4 .    Portable electrical Furnace for heating the specimen

5.    Advanced software coupling:

    •    Inverse heat transfer

    •    Mathematical Modeling

    •    Advanced FEM Suite

    •    Austenite decomposition models (JMAK, KM)

    •    Composition specific TTT diagrams and specific properties.

Comparison between ASTM D6200 and Reference QuenchProbe

Case Study 1: Comparison of different oil for same material

Case Study 2: Analysis with and without agitation

Case Study 3: Comparison of cooling curves for ASTM standard Inconel probe and Reference QuenchProbe

QuenchProbe

Case Study 4: Quench cracks in polymer quenching: 

The reason for cracking of the specimens may be attributed to excessive cooling rates at the surface compared to core for 6% polymer but not so in 14% polymer Optimum percentage of polymer required for component material can be assigned

Conclusion 4: Benefits proven by the use of Reference Quench Probe are:

1)    Compute cooling rate variation at the surface and core

2)    Compute microstructure variation at the surface and core

3)    Compute hardness variation at the surface and core

4)    Save cost – reduce destructive testing

5)    Reduce rejections, rework and ensure consistent quality always

6)    Better control of process and process stability

7)    Selection of right quench media for new components and optimum agitation required

8)    Inspection of new quenchant

9)    CQI-9 cooling curve reports

10)    Reduce defects, cracks and warping

11)    Estimate the heat transfer coefficient during quenching, and

12)    Check the effect of contamination in quenchants (water in oil; polymer in oil, etc).

Summary
Forge shops are assured of increased productivity and substantially reduced costs in hot forging and heat treatment processes by the use of protective coatings like Anti-scale protective coatings, Japanese cold-welding, synthetic die lubricants and also latest development of reference quench probe as discussed in this technical paper. A number of esteemed forge shops in India and abroad have adopted these techniques to increase productivity and substantially reduce costs.  

– S P SHENOY AND SRIKAR P SHENOY