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Material Identification and Process Verification
Material identification and process verification are essential to reverse engi- neering. This chapter will discuss the techniques used to analyze chemi- cal composition, microstructural characteristics, grain morphology, heat treatment, and fabrication processes. The chemical composition of a mate- rial determines its inherent properties. The microstructural characteristics are closely related to a material’s mechanical properties. Grain morphology reveals the grain size, shape, texture, and their configuration in a material.
These material characteristics are often analyzed simultaneously. For exam- ple, during an electron probe microanalysis, elemental chemistry is ana- lyzed to identify alloy composition; at the same time, a micrographic image will also be taken to understand the phase transformation that leads to veri- fication of heat treatment and the manufacturing process. The evolution of constituent phases in an alloy is a direct consequence of the prior manufac- turing process this alloy has experienced. The identification of these phases by their compositions and quantifying their amounts in an alloy will help engineers verify the manufacturing process used to produce the part.
The end product of material identification and process verification is usu- ally the confirmation of a material specification that is called out by the OEM in its production. Theoretically speaking, all the characteristics listed in a material specification should be tested and verified before it can be called equal to the specification of an OEM design. However, in real-life reverse engineering practice, usually only select characteristics are tested and com- pared. The characteristics that are tested are determined by their criticalities to the part functionality. The data that are specified in a typical engineering material specification will be reviewed in the next section to establish a foun- dation and create guidelines for future discussions.
products, such as engineering materials, and the processes whereby the prod- ucts are fabricated. The AMS are the most frequently cited material specifi- cations in aviation industries. In 1905 the Society of Automobile Engineers was founded, and in 1916 it joined with the American Society of Aeronautic Engineers and the engineers in other closely related professionals to form the Society of Automotive Engineers. The term automotive originated from Greek autos (self) and Latin motives (of motion), and this is a professional society that focuses on modern machinery that steers with its own power.
SAE has since played pivotal roles in the advancement of the automobile and aerospace industries (SAE, 2008).
5.1.1 Contents of Material Specification
The contents of a material specification depend on the purpose and appli- cation of this specification. A typical AMS on a product such as an engi- neering alloy is identified with a Title section on the first page, followed by eight other sections: Scope, Applicable Documents, Technical Requirements, Quality Assurance Provisions, Preparation for Delivery, Acknowledgment, Rejections, and Notes.
The Title section reveals the revision history of this AMS, the type of alloy, highlights of the material characteristics, nominal composition, and heat treatment condition.
The Scope section covers product form, such as sheet, strip, and plate, and the primary applications of this material, such as “typically for parts requir- ing strength and oxidation resistance up to 816°C (1,500°F).”
The Applicable Documents section lists all the relevant documents that form part of this specification. Two SAE publications are listed in AMS 5663, which has a composition similar to that of commercial 718 nickel alloy—
AMS 2261, “Tolerances, Nickel, Nickel Alloy, and Cobalt Alloy Bars, Rods, and Wire,” and AMS 2269, “Chemical Check Analysis Limits, Nickel, Nickel Alloys, and Cobalt Alloys”—along with seven other AMS publications. Also, two ASTM publications, ASTM E8M, “Tension Testing of Metallic Materials (Metric),” and ASTM E10, “Brinell Hardness of Metallic Materials,” are listed, along with seven other ASTM publications. Therefore, to claim the confor- mance of AMS 5663 to the OEM design data, the comparative tensile proper- ties should be evaluated in accordance with ASTM E8M (or its equivalence), which is part of the design document.
The following information is included in many of the Technical Requirements sections: composition, melting practice, condition of utiliza- tion, heat treatment, properties, quality, and tolerance. This is one of the core sections of many material specifications. The acceptable chemical composi- tion is usually tabulated with the minimum and maximum elemental con- tents specified. The acceptable analytical methods are listed in the subsection of the composition. In AMS 5663, the weight percentage of constituent ele- ments is required to be determined by wet chemical methods in accordance
with ASTM E354, or by spectrochemical methods. For lead, bismuth, and selenium, the analytical methods of APR 1313 will be utilized. The accep- tance of other analytical methods should be approved by the stakeholders in advance. In other words, the composition determined by energy dispersive X-ray analysis (EDXA) is typically not acceptable in reverse engineering to claim conformance to AMS 5663, unless otherwise agreed upon by all stake- holders. In reference to elemental composition variations, AMS 5663 requires compliance with the applicable requirements of AMS 2269. Strictly speaking, failing to meet any of these requirements can be a justifiable cause for rejec- tion in reverse engineering.
The melting practice directly affects the quality of an alloy. A specific melting practice is required for high-quality material, as is identified in its material specification. A reverse engineered product must demonstrate that it has the same melting practice as the OEM does. To claim conformance to AMS 5663, the following melting practice has to be demonstrated. The alloy should be multiply melted using a consumable electrode practice in the remelting cycle, or it should be induction melted under a vacuum. If consum- able electrode remelting is not performed in a vacuum, electrodes produced by vacuum induction melting should be used for remelting. It is worth not- ing that a double-remelting ingot is not equivalent to a triple-remelting ingot in reverse engineering either.
The mechanical property of a material is a function of its manufacturing process, and therefore the final product form. The available product forms for a material also reflect its formability and machinability, which in turn depend on heat treatment and other prior treatments. It is not uncommon to have several material specifications with the same chemical composition but different product forms and heat treatment conditions, and therefore dif- ferent material properties. The available product forms or conditions listed in the Condition subsection provide additional information for determining which material specification best fits the OEM part. For example, a material specification that provides sheet and plate product forms is a better fit for a
“heat shield” part used in a turbine engine than another material specifica- tion that only provides product forms in bar and wire.
One of the most challenging tasks in reverse engineering is to decode an OEM’s heat treatment schedule. The precise reverse engineering of a heat treatment process is virtually impossible due to the multiple parameters involved in heat treatment, such as temperature, time, atmosphere, and quench medium. It is further complicated by the fact that often several dif- ferent heat treatment schedules can produce similar material properties, but none can produce exactly the same properties of the OEM part. Many aging treatment schedules can produce the same hardness number for 2024 alu- minum alloy, but with different microstructures and fatigue strengths. Both AMS 5662 and 5663 have the same nominal chemical composition and same melting practice, and provide the same product forms, but they have differ- ent heat treatments. In AMS 5663, the precipitation heat treatment is applied
to the alloy after the solution heat treatment; however, in AMS 5662, the alloy will only be subject to solution heat treatment, although it is precipitation hardenable.
Specifically required properties are described in detail in the Properties subsection. These properties usually include various microstructural fea- tures, mechanical properties, and resistance to environmental degradation.
Two of the most commonly referred to microstructural features are grain size and second phases. The average grain size is usually measured by the intercept method of ASTM E112, which is a linear measurement. In reverse engineering it is advisable to adopt the same method of measurement of grain size for a direct comparison whenever feasible, even if a different method might give a more accurate measurement. The presence of second phases can drastically change the properties. In most nickel alloys, the Laves phase is detrimental. Both AMS 5662 and 5663 require a microstructure free of this phase, and with an acceptable amount of the acicular phase. Unless the “acceptable amount” is otherwise specified, the acceptability of a micro- structure can only be determined by a direct comparison between the OEM and the reverse engineered parts. Whenever other microstructral features are specified, such as grain texture or recrystallization, they should be com- plied with as well.
Hardness provides a first order of approximation of mechanical strength.
However, great caution is required to extrapolate mechanical properties directly from hardness. First, hardness is measured using a variety of scales, each representing different material characteristics, and there are no precise conversions among them. Second, the relationships, if any, between hard- ness and other mechanical properties are usually empirical and lack sup- porting scientific theories. These relationships are material specific with limited applicability. In reverse engineering, a hardness comparison should always be in the same scale whenever feasible. Conformance to a material specification based on hardness is an estimate at best.
What tests are required and what properties are relevant in reverse engi- neering? The short answer is that all the properties specified in the material specification are relevant for an accurate conformance. The best reverse engi- neering practice in material identification is to make a checklist, including all the relevant material characteristics and properties, and compare them item by item. This list is different for each and every material specification. For AMS 5663 it will include hardness, tensile properties at room temperature, tensile properties at 649°C (1,200°F), and stress rupture properties at 649°C (1,200°F). The reported tensile properties should include tensile strength, yield strength, elongation, and reduction, and the tensile test should be con- ducted on specimens of three orientations: longitudinal, long transverse, and transverse. A word of caution: many material specifications only list the required minimum tensile properties, as shown in both AMS 5662 and AMS 5663. These two specifications require identical tensile properties despite different heat treatments. AMS 5663 requires solution treatment followed by
precipitation hardening, while AMS 5662 only requires solution heat treat- ment. If a tested tensile strength meets the AMS 5663 minimum require- ment, then it does literally satisfy the specification requirement. However, unless a direct comparison with an OEM part, it cannot be decisively con- cluded that it has a tensile strength equivalent to that of the OEM part. The OEM part might have a tensile strength above the minimum requirement. In reverse engineering, the baseline material properties for comparative analy- sis are the test results directly measured from the original part, not a mate- rial specification.
The tolerance requirements depend on part shape, dimensions, and other factors, such as material flexibility and deformability. In AMS 5662 and AMS 5663, the requirements of tolerances are simply summarized as “all appli- cable requirements of AMS 2261.” However, different requirements might be required in other cases.
The Quality Assurance Provisions section summarizes the responsibility of inspection and classification of tests. Ideally, each heat or lot is tested, and their respective microstructures examined to ensure high quality control. The sam- pling and testing should also comply with proper procedures, such as AMS 2371: “Quality Assurance Sampling and Testing Corrosion and Heat-Resistant Steels and Alloys Wrought Products and Forging Stock.” Any product not con- firming to the specification should be rejected, and another alternate material should be considered in reverse engineering.
Specifications on specific subjects are also published. For example, AMS 2242 and AMS 2262 focus on tolerances. They cover established manufactur- ing tolerances applicable to various product forms made of different alloys.
These specifications provide a good reference and guidance on reverse engi- neering where manufacturing tolerances are often of great concern. Another example, AMS 2248, focuses on chemical analysis limits. It defines limits of variations for determining acceptability of chemical composition of a variety of parts, and provides a valuable reference in alloy composition determina- tion, where the acceptability of variation limits often challenges engineers.
Justifications are required to adopt any tolerance or composition if it is out of the scope of the specified ranges.
The best way to confirm a material specification in reverse engineering is by direct comparison of each and every characteristic listed in the specifica- tion. However, an alternate method of compliance might be acceptable upon approval or mutual consent.
The material specification goes beyond composition identification and manufacturing process verification. It also extends to packing and identifica- tion. AMS 2817 covers procedures that will provide protection for preformed packings of O-rings of elastometric materials from contamination by foreign materials prior to installation, and ensure positive identification. Part pack- ing and identification, though of an administrative nature, also play a crucial role in a reverse engineering project to avoid preinstallation contamination or damage.
5.1.2 alloy Designation Systems
Many alloy designation systems have been developed by various organi- zations such as SAE International and ASTM International. Different alloy codes and standards are also published in different countries, such as British Standards, German DIN, Swedish Standards, Chinese GB, and Japanese JIS.
DIN stands for Deutsches Institut für Normung in German, and German Institute for Standardization in English. It is the German national organiza- tion for standardization. The DIN EN number is used for the German edi- tion of European standards. A Swedish Standard is usually designated with a prefix SS. The GB standards are the Chinese national standards issued by the Standardization Administration of China. GB stands for Guobiao, a phonetic transcription of the word National Standards in Chinese. JIS stands for Japanese Industrial Standards. It is published by the Japanese Standards Association.
It is of great advantage to have a universally unified alloy code system; that is why the Unified Numbering System (UNS) was proposed. This system consists of a prefix letter and five digits designating a material composition.
For example, the prefix S is used to designate stainless steels. UNS S31600 is the unified code in the Unified Numbering System for one of the most widely used stainless steels, which is designated as SAE316 by SAE International, 316S31 in British Standards, and SUS 136 in Japanese JIS. However, in the European system, it is designated with a DIN EN number of 1.4401, and given a name of X5CrNiMo17-12-2, while the Swedish Standards system designates it as SS2347. A comprehensive cross-reference system between the UNS and other alloy code systems is yet to be established. From a reverse engineering perspective, the biggest concern is whether two nominally equivalent stain- less steels coded in different systems are actually identical. A UNS number alone does not constitute a full material specification because it establishes no requirements for material properties, heat treatment, product form, and quality. Several material specifications are published for stainless steel 316:
AMS5524, ASTM A240, and ASTM A666. Great caution needs to be exercised when drawing any inference from cross-references based on different desig- nation systems or codes.
5.2 Composition Determination