Equation for the Successful Design of Heat-Resistant Fabrications
Fabrications used in the heat-treating industry are exposed to harsh environments including elevated temperatures, thermal quenching, aggressive atmospheres and more. Individually, they can shorten useful life of the product and add substantially to maintenance costs. Combined, these harsh environments can create havoc with the bottom line of any heat treater.
|Fig. 1. Qual-Fab facility|
|Fig. 2. Equation for successful ally design|
Components fail for a variety of reasons, some of which are predictable while others are not so obvious. Three of the most important items to consider when designing a product are (Fig. 2):
- Knowledge of operating parameters
- Familiarity with materials available in the marketplace
- Experienced fabrication techniques
|Fig. 3. Radiant-tube failure|
When asked what the operating temperature of the furnace is, the answer will usually be forthcoming. The problem is that often the wrong question is being asked. In many furnaces, there can be large temperature gradients within the furnace itself. In radiant-tube-fired furnaces, for example, the temperature of the tube surface may well exceed the operating temperature of the furnace by several hundred degrees (Fig. 3).
The same holds true for a muffle-tube-style furnace, where the temperature of the muffle tube itself can greatly exceed the operating temperature of the furnace. This added temperature almost always has a considerable effect on the material that should be used to fabricate these items to give proper service life.
Introducing atmospheres into the furnace can cause thermal shock to a product, which results in thermally induced stress. Gases with small amounts of sulfur are very detrimental to nickel-bearing alloys. If you see a green scale deposit on your hearth or tube, you might want to examine the atmosphere being introduced into the furnace or look at the cleanliness of the parts being heat treated. Residual oil from a machining operation can also be a culprit in premature failure of many materials.
Quench temperature and media are also important factors that affect the life of a heat-treat basket. Proper part loading and weight distribution is an important parameter in keeping the operating stresses in furnace base trays within the limits of the design. Knowing and controlling all operating parameters will go a long way in extending the life of your heat-treat fabrication.
|Fig. 4. 330 pyrolyzer failure|
In today’s highly competitive marketplace, it is important for every heat-treat company to get the maximum bang for each maintenance buck. The issue of cost versus service life is paramount in making material decisions.
Alloy selection has a significant effect on both the service life of the heat-resistant fabrication and on the initial cost of the item. As a rule of thumb, material costs represent 50-70% of the purchase price of a furnace component. Depending on the additional costs of maintenance and furnace downtime for each application, considerations regarding the use of the best alloys are often warranted. Working with a fabricator that is experienced in the use of these alloys can be very helpful in making these types of purchasing decisions.
Knowing what’s new in the marketplace is another benefit that an experienced supplier offers. Thirty years ago, Inconel 601 was targeted at the muffle and radiant-tube markets. Today, materials like RA 253 MA® are being utilized to provide the performance of more highly alloyed material while providing a lower initial cost due to leaner chemistries. To successfully select an alloy that can withstand extreme environments as well as provide initial cost savings, it is important to have a thorough working knowledge of the application where the metal will be placed (Fig. 4).
For successful material selection, it is almost never possible to design based on temperature considerations alone. In addition to knowing the temperature variations in the furnace, it is also important to understand what potentially corrosive gases will be circulating in the furnace environment, as most every alloy will react differently. This does not only apply to the gases that are injected into the system but also the by-products of the heat treatment that could off-gas while in the high-temperature application.
One final concept to take into consideration is how the material will be cycled. Constantly heating and cooling the material could pose the potential for the material to fail due to constant expansion and contraction. To increase the longevity of the alloy, it is best to keep temperatures consistent so that constant thermal cycling is not an issue (Fig. 5).
|Fig. 5. Inner cover expansion failure|
Completing the equation is working with an experienced alloy fabricator that is knowledgeable in the proper methods to cut, form, weld and machine high-nickel alloys and stainless steels. With the right knowledge, the fabricator can provide legitimate shortcuts for material processing and fabrication to provide maximum cost benefit.
- Cutting: Laser and high-definition plasma cutting are now being utilized in the fabrication process. They offer the benefit of producing optimum-size blanks with minimal distortion to minimize additional work by the fabricator.
- Forming: New hydraulic-press brakes provide a new freedom and control in the forming process. Tonnage is the secret to proper forming, and newer modern machines provide this asset to help the fabricator.
- Welding: Most heat-resistant alloy products are still being joined using the tried-and-true methods of GMAW and GTAW welding procedures. Fabricators are utilizing semi automatic and automatic welding equipment where products can be readily fixtured.
In addition, the experienced alloy and high-nickel fabricator can often offer suggestions or costs to rebuild some of the parts traditionally consumed in the heat-treat shops by reusing some of the materials previously unaffected in the failure.
A strong relationship between a heat treater and an experienced and qualified stainless and high-nickel-alloy fabricator, applying the parameters of the referenced equation, is the surest way to reduce your operating costs. IH
For more information: Contact Gary Vanek, vice president, Qual-Fab, Inc., 34250 Mills Road Avon, Ohio 44011; tel: 440-327-5000; fax: 440-327-5599; e-mail: firstname.lastname@example.org; web: www.qual-fab.net