Making the Difference in Heat Treat
With most manufactured gears requiring heat treatment, you can see why the heat treater plays such an important role. The gear manufacturer has to put much trust in the operations performed by the heat treater. This is where an in-house heat treat can make the difference.
The KeyIn-house heat treat is the key to the consistency in results from the pilot run to the production run. What makes all this possible is the use of various controls during the heat-treat process. Controlling the fixturing of the loads, the use of programmable controllers and requiring certified instrument calibrations are all examples of these controls. Having control of the process enables us to duplicate the identical heat-treat pattern every time we run the same parts. Hero Motors sees this in-house heat-treatment difference as a benefit to our customers and to us (Fig. 1).
Five important controlling factors that make a difference in our heat treat are:
- Complete and thorough documentation of all test and production runs
- Proper fixturing of special loads
- The use of modern and sophisticated instrumentation
- A regularly scheduled preventive-maintenance program
- Qualified outside sources that calibrate and certify our instruments and hardness testers
Complete and thorough documentation of the parts to be processed is very important. We begin each component by issuing a work order. On this work order is all the information needed to process the parts through the heat treat, including any special instructions like special fixturing. Once the job is in process, documentation of the heat-treat cycle is recorded. This information lists important factors such as the time required for the furnace to get up to heat, the length of the diffusion cycle, what type of quench was used and the tempering temperature. The process is considered established once two heat-treatment cycles give the required results.
We also keep a log of all our furnace charts. Once a load enters the furnace, the operator is required to record the date, part number, his name and number of pieces in the load on the chart. The chart gives us a visual record of the cycle time and the furnace temperatures. This data is also fed onto the SCADA (Supervisory Control and Data Acquisition) system. Tracking and archiving of all important process parameters are performed through the SCADA (Fig. 2).
One critical control at the hands of the heat treater is the loading pattern of the furnace load. How you make up your load makes the difference. You can speed up the quench or slow it down simply by adding or deleting parts. It is vital that you make up your load the same way every time to obtain the same results. All special-fixturing instructions are noted on the work instructions. We also ensure that full loads are processed every time. Scrap is used if there are not enough parts to fill the load (Fig. 3).
The use of modern and sophisticated instrumentation is another way we control the heat-treat process. Oxygen probes are used in our furnaces to maintain a consistent carbon potential. The probe and its controller regulate the carbon potential automatically. We also have a CO analyzer to ensure that the CO levels are maintained in the furnace (Fig. 4).
The controlling of the furnace is done by the concept of “recipe.” The recipe is fed onto the PLC, and the PLC gives data to the controllers. Eurotherm controllers enable us to program the furnaces. In this way, we ensure the same process every time: time at heat, diffusion cycle, soak time and quench time and temperature. Controllers provide repeatability and accuracy.
A regularly scheduled preventive-maintenance program is essential in controlling the heat-treat process. The equipment must work properly in order for our controls to work. Performing each and every preventive check at the proper time ensures that the equipment works properly. Checks are performed daily, weekly, monthly, quarterly, semiannually and annually for all equipment. Personnel are trained in all these activities.
Half-yearly checks of the thermocouples in all our furnaces are also performed against a master thermocouple. This test measures the accuracy of the thermocouples being used. The thermocouple is replaced once a year.
Certified outside sources calibrate our equipment. Each and every controller is calibrated once a year. Once a year, they conduct a complete temperature uniformity survey, placing a fixture consisting of nine thermocouples inside the furnace. These thermocouples measure the heat at the top, bottom, sides and center of the furnace, making sure there is a uniform heat pattern. Even the quenching oil is checked quarterly for parameters like water content, viscosity, etc.
We also do metallurgical profile checking twice a year wherein 11 test pieces are placed in a charge and the metallurgical deviation noted to ensure an even pattern. A record of this testing is kept in the SCADA.
An NABL-certified laboratory calibrates and certifies all of our hardness-testing instruments twice yearly per the international standards. A record of all the calibrations performed with the data is kept in the SCADA to have a concise statement of all the past calibration records.
SummaryWith the combination of these five controls and others, which are specific to our business, Hero is able to combine our gear-manufacturing capabilities with our in-house heat treat to produce precision, quality gearing. Having an in-house heat treat with an experienced team makes the difference for us as a gear manufacturer. We are able to work side-by-side controlling the manufacturing and heat-treating processes, resulting in quality end products. IH
For more information: Raman Ramchandran is general manager (Gear Project) for Hero Motors Ltd., 10th Km Stone, G.T. Road, P.O., Dujana - 203207, Dadri, Distt. G.B.Nagar, U.P. (India); tel: +91-0120-2674207; fax: +91-0120-2674557; e-mail: firstname.lastname@example.org; web: www.heromotors.com. Mr. Ramchandran was assisted by M/s Dharmender Kumar, Amit Chauhan, Ajay Kumar.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: oxygen probe, carbon potential, diffusion, SCADA, temperature uniformity survey