Heat-treating metals is truly all about the details. There are many variables that affect the final properties resulting from the heat-treatment of steel; steel composition, hot work ratio, the heat-treat process assignment, execution of the process, hardness testing accuracy, equipment condition, and weather, to mention just a few. Yes, the seasonal change in weather can actually affect the temperature inside an industrial heat-treating furnace, resulting in variations in hardness, yield and tensile strength, and increasing rework rates. That can result in increased costs of processing metals in the industrial manufacturing process.
Please don’t let the change of season and BTU values affect your results and profits!
Each of the variables listed above have several components within them that may also vary. We can control or influence many of these variables to a degree; the rest we must either endure or challenge. We can’t control the seasonal change in weather, but we can plan for how it affects the heat-treatment of metals in industrial manufacturing.
Consistency in a heat-treat operation is the single most important thing to build. From procedures on how orders are written up for heat-treating to how equipment is maintained, to how consistent your loading practices are… it all matters.
For those of us heat-treating alloys with steep tempering curves, precision in the temper process is critical. There is a seasonal effect that many heat-treaters overlook, yet it can increase your rework rates remarkably twice a year if you’re not aware.
When the weather changes, the colder regions of the United States experience a large increase in demand for heat to keep homes and workplaces warm. Natural gas is by far the most used fuel for heating in the continental U.S. Texas is by far the largest natural gas producer in the U.S. and as demand increases, much of that gas is needed by those folks in colder areas. Pipelines are the most common method of transportation for natural gas, and companies charge by the cubic foot to transport gas in their pipelines. Since heat energy is really the value in natural gas, it is sold by the ‘Therm’ (100,000 BTU).
The BTU content of natural gas actually varies quite a bit; a low BTU natural gas may have as little as 983 BTU per cubic foot, while a high BTU natural gas may have as much as 1,090 BTU per cubic foot. Though 10% may not seem like much, the energy sellers can save a lot of money transporting high BTU natural gas instead of the low BTU gas. As a result, the high BTU gas gets piped to the North, and in the South we get the lower BTU gas because of the lower transportation costs.
That’s a lot of information just to tell you that the natural gas we get in the South in the winter can have up to 10% lower BTU content than the natural gas we use in the summer. Since industrial furnaces and ovens are all set up on flow rates based on cubic feet, a furnace could conceivably use 10% more gas in the winter than in the summer.
How can this affect the temperature inside a furnace? After all, there is a temperature controller that controls the temperature by adjusting gas and air flow into the furnace. Well, furnaces run on a mixture of gas and air, and the BTU content of air doesn’t change appreciably at a fixed location. So, the mixture of air and gas moving through the control valves in the winter has a different BTU value than it does in the summer. The gas regulators, proportionators, and control valves pass through the same volume of air and gas, but with much different heat content, so the furnace performs differently. The difference is small, but the hardness that results can be the difference between a 235 and a 241 HBW (Brinell Hardness Number), and that can be the difference between a passing result and rework. Of course, there are also related changes in yield and tensile strength.
Unless you keep very detailed records and have tight processes, you may have had this problem and never realized the cause of it. If your rework goal is zero percent and you are already watching the chemistry of your incoming product, making temperature adjustments accordingly, and have rigorous procedures in place, then this is something you’ll want to watch for. Once you notice the effect, you may want to apply a few degrees of offset to your tempering processes, but don’t go overboard, and be sure to document changes and evaluate your results.
The effect is more pronounced when winter weather arrives and the BTU content of natural gas is reduced in the warmer regions of the country. Of course, when warm weather returns for all, the BTU content increases again in the South, but the effects of the change are less sudden and less dramatic to metals heat-treat processing than at the onset of winter.