Heat treatment is an energy user and all too often an energy abuser. Given the significant amounts of energy needed to run a typical process, this translates into a huge cost to the heat treater. In addition, material choices and specification requirements often dictate cycle temperatures and times, while environmental compliance further adds to the overall cost. How can we, and how should we, go about addressing these issues? Let’s learn more.

Energy ^[1,3,4]

The first step in any energy-management program is to understand where and how much energy is being consumed or wasted. Energy monitoring via gas totalizers or electrical power meters (including peak-demand monitoring) is a good way to start. Thermal imaging (Fig. 1) is another tool to help focus maintenance activities and identify areas for improvement.

A complete energy audit normally consists of five basic parts: ^[1]

• A description of the facility and all of the equipment involved
• Mapping of the current energy consumption in the plant
• The short-term energy consumption in the plant
• The long-term energy consumption in the plant
• A search for energy-efficiency improvement possibilities

By way of example,^[1] an energy audit was conducted at a large commercial heat treater in Europe with the following major findings.

• The plant consumed 9,590 MWh/year of electricity and 120 MWh/year of peak demand costs at the start of the audit.
• 68% of the energy was consumed by primary heat-treatment processes and equipment with the other 32% consumed in ancillary processes.
• The largest portion of the energy consumption was the main 14 integral-quench-style heat-treatment furnaces. The ancillary equipment consisted of over 30 units (preheating and tempering furnaces, parts washers).
• The electricity consumption could be decreased by 7.85% (753 MWh/year) by energy conservation (housekeeping and recipe optimization) measures.
• The electricity consumption could be further decreased by 4.36% (418 MWh/year) and the peak demand eliminated by energy monitoring and energy-saving investment measures.
• The combination of the proposed energy housekeeping measures and energy-saving investment measures reduced the energy cost with savings of approximately $135,000 per year. It also resulted in a reduction of the carbon-dioxide emissions by approximately 740 tons per year.
• If all proposed measures were implemented, the total investment cost would have a payback of three years and a net present value of almost $2.1 million over 10 years using current energy prices.

Environmental

Environmental compliance is a complex subject. I’ve often seen heat treaters shaking their heads in disbelief as to why they must both pay for water and pay a sewer tax to dispose of it having done nothing more than heat it a few degrees. These types of issues have led to a philosophical debate within the heat-treatment industry as to what constitutes good environmental policy. Stark contrasts can be seen in different countries throughout the world.

In the U.S., for example, environmental policy is viewed as obeying the rules and regulations mandated by federal, state and local governmental agencies. By contrast, many corporations strive to be “good neighbors,” often imposing more stringent environmental policies than those mandated by law.

In Europe, there is a strong cultural emphasis placed on environmental issues, which are at the forefront when considering equipment purchases and the way in which heat-treat plants are managed. Emissions are not only carefully monitored, but companies often strive to outdo one another so as to be rewarded rather than penalized (e.g., the carbon tax) for their environmental choices.

In Asia, as societies continue to evolve, environmental policies will follow suit. For example, one heat treater in Japan has coy fish swimming in his cooling-water system, and each employee is responsible for keeping his fish alive and healthy! By contrast, the smog that surrounds cities in China is a clear indication that we still have more work to do.

Process Optimization

Traditionally, when a load is heat treated in a furnace, the only measurements available to track the cycle progress are furnace control thermocouples or, in some instances, part workload thermocouples. The goal is to heat the entire load to a specific temperature and soak at that temperature for a given period of time.

Ideally, the user would like to know the temperature at the center of the load and the moment that all of the piece parts reach that temperature. If known, cycle times can be optimized. Most heat-treat recipes soak the load longer than necessary to ensure meeting metallurgical and mechanical properties. This adds cost and reduces throughput.

There are currently three principal methods for ensuring a stable and uniform load temperature.

• Manual intervention – visual inspection of the load during processing. This can be done by observing the load through a peep sight or, in some rare instances, by opening the furnace door(s). This method is dependent on the skill of the operator and can disrupt furnace operation.
• Standard automatic control – where the heating process parameters are calculated and the soak time is over-specified to ensure uniformity.
• Process supervision to monitor energy consumption as well as all of the usual parameters, ensuring that the required heating practice is achieved without wasted time and energy.

Companies are creating modeling algorithms and so-called visual supervisors ^[2] that calculate furnace parameters, allowing for improved temperature uniformity. This in turn allows the cycle time (regardless of the load) to be optimized. This steady-state method results in reduced cycle times and energy costs.

Finally, optimization of all diffusion-based processes (Fig. 2 -
online only) can be achieved by increasing process temperature as a means of reducing cycle time. In this case, cost reduction can be achieved by such methods as:

• Modifying material chemistries
• Improving process control to minimize case-depth variation
• Controlling distortion to avoid post-heat-treatment machining operations
• Choosing more-efficient diffusion processes for the application

Cost ^[1]

Cost containment is a huge issue for the heat treater. The net present value is the difference between the investment and the present value of future cost savings due to the investment. The present value of future cost savings is calculated with an interest rate, usually the cost of capital. The net present value is calculated according to:

where NPV is the net present value, t is the time frame for the calculation, a_i is the annual cost saving due to the investment, r is the cost of capital and I is the total investment cost.

This method considers the time value of money and accounts for all cash flows during the time frame for the calculation. It can be used to evaluate an investment and to compare the profitability between investments. A good investment will have a high net present value.

Another way to compare investments is to calculate the net present value ratio, which the net present value is divided by the initial investment according to:

where NPRV is the net-present-value ratio, NPV is the net present value and I is the total investment cost. A higher NPRV indicates a more profitable investment.

Summary

In order for heat treating to be the most cost-effective solution to manufacturing, we must continue to evolve in the areas of technical innovation; improved up-time productivity; and reduction of energy, environmental and process costs.

Simply reducing cost by heretofore traditional methods (e.g., labor reduction, quality relaxation, deferred reinvestment or delayed introduction of new technology) will no longer keep the industry competitive. Applying conservation methods and negotiating more-favorable energy contracts are a good start.