The Role of the Heat Treater in Reverse Engineering
Reverse engineering is a true journey of discovery to uncover the fundamental principles of a component through analysis of its functionality, properties and microstructure. Such analysis can then be used to redesign the component usually in a reduced time frame, using such tools as computer-aided design modeling and rapid prototyping. This process typically involves a small number of highly specialized components. Speed and flexibility over traditional design and manufacturing methods are the hallmarks of reverse engineering.
Why Reverse Engineering?Corporations often benchmark their products against those of competition by systematic disassembly and evaluation of the function of each component. Reverse engineering is also commonly used in the aerospace industry to produce replacement parts when the original source no longer exists, when the product has become obsolete or when the original product design documentation has been lost (or never existed). Reverse engineering can also correct previous mistakes or make a good feature into a great one. The goal of reverse engineering is to save cost by reducing analysis time and testing. Perhaps the biggest challenge faced by this process is the determination and duplication of the heat-treated microstructure.
The ProcessThe steps involved in this process include:
- Form-fit-function analysis
- Application review
- Material identification
- Mechanical-property determination
- Heat-treatment determination
- Confirmation testing
1. Examine the product. Understand (predict) how the product should work or how the product does work before disassembly.
2. Gather and analyze (interpret) all available information including customer requirements. With government projects, there may be overhaul manuals that define some or all of the critical dimensions and tolerances.
3. Acquire parts to use for dimensional data (e.g., size, shape, fits, interface considerations, etc.) and evaluate types of conditions to which the reverse-engineered component will be subjected. For wear issues, concerns should be part loading, seal surface condition, fluid erosion, etc.
4. Inspect and record all dimensions of the assembly.
5. Document the assembly with digital photographs.
6. Create a plan. Disassemble and document each part. Pay special attention to areas of wear, coatings, etc.
7. Develop a detailed understanding of the actual product function (structure) based on the disassembly process.
8. Establish primary engineering data.
9. Inspect each part visually and optically (via stereomicroscope). If appropriate, use other NDT methods.
10. Create a materials/components list (bill of materials).
11. Apply engineering analysis, simulations and build models (prototypes). Create preliminary drawings of each part and the assembly.
12. Establish tolerances from statistical analysis of measurements (if there are sufficient samples to inspect) or preferred standards.
13. Manufacturability review of drawing.
14. Perform chemical analysis to determine type(s) of material(s) in use.
15. Perform metallurgical, mechanical and physical testing to establish mechanical properties, thermal treatment and coatings. Results should include surface treatments and hardness, case depth (if applicable) and core hardness.
16. Deduce heat-treatment processes including pretreatments, localized effects, heat treating and mechanical working.
17. Deduce other manufacturing processes that will impact properties or end-use performance.
18. Examine metallurgical structure for any grain size, shape and grain flow.
19. Revise drawing with material chemistry, heat-treat recipes and specifications.
20. Produce finished components and perform functionality and life tests.
The engineering drawings created will provide dimensions, tolerances, finishes and other critical requirements. Function, however, is not normally specified on drawings or even referenced in specifications, so it must be determined by performance testing that simulates the mechanical, physical, metallurgical and environmental requirements of the end-use application. This requires an understanding of the materials, processes and heat treatments as well as gathering historical information, supplier records and material callouts.
General notes on drawings may help if they specify manufacturing operations. For example, drawings may call out method of cleaning, type of heat treatment, type of material, surface treatment (e.g., coatings, shot peening) surface finish, ancillary processes (normalizing, annealing, stress relief), inspection (penetrant or magnetic particle, etc.), process sequence (heat treat after welding, inspection after heat treatment), tooling (fixtures, etc.) and quality control tests (magnetic particle inspection, etc.) Preferred suppliers may also be called out indicating where to obtain specialized components and difficult-to-procure materials or where to go for specialized processing (casting, brazing, plating, straightening).
In the world of reverse engineering there is a simple cost equation:
(1) Cost = Material + Manufacturing + Inspection + Finishing + Rework
The ApproachUsing a sample component known to be of good quality is a luxury seldom afforded the reverse engineer. Many times the component delivered to us is worn, damaged or abused in some fashion. This makes the job much more difficult. Part dimensions determined by direct measurement must be altered to take into account these factors. Material must be determined by chemical analysis, but oftentimes subtle issues such as trace-element chemistry, grain size and prior microstructure must be factored into the analysis. Selection of the proper mechanical-test methods is a critical component of the process. One must also realize that results may vary, and validation is an important aspect of the final determination.
Mechanical testing helps to determine and/or confirm the heat-treatment methods used. Drawings, if available, typically call out hardness and perhaps key strength or ductility values as a range or a minimum. Indirect mechanical-test methods (preferred due to their cost) include hardness and conductivity testing, while direct methods include tensile testing, impact and torsion testing and corrosion tests.
Hardness alone cannot be relied upon to provide the complete picture to determine heat-treatment details. For example, SAE 4340 bar can be specified in accordance to MIL-S-5000 (air melted) or MIL-S-8844 (vacuum melted). Either form produces identical tensile properties and hardness. However, MIL-S-8844 has superior toughness and low temperature properties. Similarly, Inconel 718 sheet can be purchased per AMS 5596 or AMS 5597. While both have nearly identical tensile properties, they require different heat treatments and demonstrate different creep properties.
Heat-Treatment PrecautionsHeat treating requires both a process and a piece of equipment in order to be successful. Controlling both process and equipment variability is the key to ensure that the design engineer gets the product response he is expecting. One of the dangers encountered when reverse engineering a product is to not take into account the service history. Many projects fail due to a correct heat treatment to an incorrect specification. Finished effective case depth, for example, is not the same as effective case depth. Also, international standards, global material sourcing and advanced heat-treatment methods and equipment in use today may not have been in place when the component was originally manufactured.
Example 1: Ball Nut (Fig. 1)
This B1 Lancer slat-actuator ball screw is made from SAE 4150 material. Examination of this item revealed that it was induction hardened after preliminary groove cutting. The scalloped pattern of the induction-hardened area resulted from the material removal that increased the distance to the coil.
This P3 Aileron control chain is a carburized component. The pin was carburized to provide a wear surface. After installation, the ends of the pin were swaged to retain them in the assembly. Subsequent analysis determined that the ends of the pin were induction annealed to allow the swaging without breaking or cracking of the edges.
The C5 Galaxy flap jack screw has carbon-enriched “tips” followed by an anneal from carburizing temperature. It is then induction hardened followed by edge tempering. The core was to remain very soft so that it conforms to a helical ball-screw groove in the nut and does not damage the groove in the screw.
From the C5 Galaxy, this slat-actuator ball screw is an example of an unexpected process – namely carburizing. The reverse-engineering assumption was induction hardening. However, the chemistry and metallurgy supports carburizing per AMS 2759/7. Examination of this item also revealed that it was partially machined before carburizing.