5.3 Microstructure Analysis
5.3.1 Reverse Engineering Case Study on Ductile Iron
most EDXAs partially results from the interaction between the incident elec- trons and the outermost-shell valence electrons of the sample atoms, which slows down the speed of the incident electrons and releases their kinetic energy to form the background noise. WDS is a high-quality technique com- monly used for quantitative spot analysis. EDXA shows a spectrum of ele- ments of a sample simultaneously, as illustrated in Figure 5.7a and b. WDS, however, can only read a single wavelength and analyze one element at a time. The X-ray intensity in any quantitative analysis should be corrected for the matrix effects associated with atomic number (Z), absorption (A), and flu- orescence (F), the so-called ZAF factor. In reverse engineering applications, EDXA and WDS are best used as complementary analytical tools. EDXA can be used first to scan the general chemical makeup of an unknown sample, and then WDS is applied to more accurately conduct a quantitative analysis of specific constituent elements of the sample.
It provides a wealth of information on alloy classification and heat treatment parameters. Based on its distinctive microstructure, if a part is made of duc- tile iron, its verification can be easily confirmed in reverse engineering. The nominal chemical composition of ductile iron is listed in Table 5.2, though a chemical analysis is required to accurately determine its exact composition.
Compared with steel, which contains less carbon and silicon, usually 0 to 2%
each, ductile iron has noticeably higher carbon and silicon contents, which along with magnesium will lead to the formation of nodular graphite, as shown in its trademark microstructure. In fact, the weight percent (wt%) of carbon and silicon contents is used as an index for ferrous alloy classifica- tion, which is referred to as carbon equivalent (CE), CE = wt% C + 0.33 wt%
Si. Seven different types of graphite morphology are defined in ASTM A247 (ASTM, 2006a). The graphite appears in thin flakes like potato chips in gray iron; but in malleable iron, it appears in a massive bulk form like popcorn.
Most carbon is consolidated as iron carbide and pearlite in white iron. These microstructure morphologies provide convenient and convincing evidence for alloy identification as ductile iron, gray iron, malleable iron, or white iron, and the affiliated heat treatment schedules that produced them.
More advanced and accurate elemental analyses might be required for the critical part made of ductile iron. Each element has its own specific effect on ductile iron, as detailed below, and its content might need to be determined precisely to meet the specific quality control requirement. Silicon is a heat- resistant element and is usually added for high-temperature strength and to induce graphite and ferrite formation. However, the toughness could dete- riorate to an unacceptable level when silicon is higher than 2.6%. Manganese
FIgurE 5.8
Microstructure of a typical ductile iron with nodular graphite surrounded by ferrite in a matrix of pearlite.
promotes the formation of pearlite and enhances hardness and strength, but it decreases the annealability and increases segregation. Usually the content of phosphorus is kept as low as possible, since a 0.01% increase in phospho- rus could reduce ductility by 1%. The toughness could also deteriorate to an unacceptable level when the content of phosphorus is higher than 0.02%.
Sulfur could impede the formation of nodules and “de-ductile iron” the alloy.
Magnesium is a deoxidizer: It is added to remove sulfur and produce nodules.
However, excessive magnesium could cause graphite to explode and produce shrinkage and spikes in graphite that are sometimes undesirable. Chromium produces carbide. The addition of cerium could tie up tin, vanadium, tita- nium, lead, and aluminum to isolate these undesirable elements.
The properties of ductile irons of similar compositions are heavily depen- dent on processing, heat treatment, casting section size, and the subsequent microstructure. The reverse engineering of ductile iron is beyond just identi- fying its composition. To confirm the grade and properties of the ductile iron used for the OEM part, more comparative analyses are needed. Several stan- dards have been documented for the properties and specifications of ductile irons. ASTM A897 and its metric version, 897M, list the mechanical property requirements of austempered ductile iron in different grades, as summarized in Table 5.3 (ASTM, 2006b). The properties described in metric and U.S. cus- tomary units are rounded up for easy comparison. Hardness is only listed in ASTM A897 for reference, and is not mandatory. The critical properties of each grade of ductile irons are dependent on their intended applications.
For example, 1600/1300/–grade ductile iron has an ultimate tensile strength of 1,600 MPa (232 ksi) and a yield strength of 1,300 MPa (189 ksi). It does not specify tensile elongation and Charpy impact toughness because it is primar- ily used for gear and wear resistance applications. Grade 1400/1100/1 sacrifices some wear resistance to improve ductility and toughness. It has an ultimate tensile strength of 1,400 MPa (203 ksi) and a yield strength of 1,100 MPa (160 ksi), and requires a tensile elongation of 1%. It is also used for similar TablE 5.2
Typical Chemical Composition (Weight Percentage) of Ductile Iron
Major Elements Trace Elements
Carbon 3.6–3.75 Chromium 0.06 max
Silicon 2.2–2.6 Cerium 0.005–0.015
Manganese 0.35 max Tin 0.01 max
Phosphorus 0.02 max Vanadium 0.02 max
Sulfur 0.02 max Titanium 0.02 max
Magnesium 0.030–0.050 Lead 0.004 max
Copper 0.10–0.80 Aluminum 0.05 max
Nickel 0.10–2.00 Other <0.01
Molybdenum 0.0–0.20
Source: Data from the Ductile Iron Society.
applications. With the same chemical composition, different properties will result from different heat treatments, such as austempering. Austempering is a heat treatment process that heats ductile iron castings to the austenitizing temperature, usually between 816 and 927°C (1,500 and 1,700°F), depending on the grade, and holds at this temperature long enough to dissolve the car- bon in austenite. The part is then quickly quenched to 232 to 399°C (450 to 750°F). The quenching time is usually just a few seconds to ensure a sufficient cooling rate. It is critical to avoid the formation of pearlite around the car- bon nodules during quenching, as this would reduce mechanical properties.
Afterwards, the part is held at the austempering temperature for isothermal transformation to form a microstructure of acicular ferrite in carbon-enriched austenite. Figure 5.9 schematically illustrates a typical austempering process with unspecified parameters, such as austenitizing temperature, holding time, quenching medium, and cooling rate. To duplicate an equivalent part using reverse engineering, these heat treatment parameters should mirror the parameters in the OEM’s process as much possible. The reference docu- ments that verify an OEM’s heat treatment parameter, such as the austem- pering temperature, are not always available in reverse engineering. They sometimes rely on corporate knowledge, information from a professional society such as the Ductile Iron Society, or test results.
Through proper control of the austempering conditions, design engineers can produce a range of properties for austempered ductile iron (Keough, 1998). For instance, for high ductility, good fatigue and impact strengths at the sacrifice of yield strength, a higher austempering temperature is used.
A test showing a yield strength of 496 MPa (72 ksi) implies that the austem- pering temperature is most probably around 399°C (750°F) using the duc- tile-iron-grade scale as a reference. On the other hand, a lower austempering temperature is used for applications requiring a higher yield strength, TablE 5.3
Mechanical Property Requirement of Austempered Ductile Iron
Grade
Ultimate Tensile Strength, MPa (ksi)
Yield Strength, MPa (ksi)
Elongation (%)
Brinell Hardness Numbera
Charpy Impact Value,
Joule (ft-lb)b
750/500/11 750 (109) 500 (73) 11 269–321 110 (81)
900/650/9 900 (131) 650 (94) 9 269–341 —
1050/700/7 1,050 (152) 700 (102) 7 302–375 80 (59)
1200/850/4 1,200 (174) 850 (123) 4 341–444 60 (44)
1400/1100/1 1,400 (203) 1,100 (160) 1 388–477 35 (26)
1600/1300/— 1,600 (232) 1,300 (189) — 402–512 —
Source: ASTM A897-06 and A897M-06.
a Typical range; not a required specification.
b Average of the highest three test values of four test samples of unnotched Charpy bars tested at 295 ± 4K (70 ± 7°F).
hardness, and good wear resistance. If the test shows a yield strength of 1,379 MPA (200 ksi), the austempering temperature is most likely around 260°C (500°F). Based on the mechanical properties of the OEM parts, the engineer can use either the documented reference data or the actual test results from simulated samples to figure out the austempering tempera- ture. The tensile property requirements of ductile iron castings are listed in ASTM A536 (ASTM, 2009).
The property requirements of automotive ductile iron castings are summa- rized in SAE J434C. In contrast to ASTM A897, the characteristics of ductile iron are primarily specified by hardness and (micro)structure in SAE J434C for automotive industries, and automotive engineers often specify the hard- ness value in their procurement. This is technically acceptable because both the ultimate tensile strength and yield strength of ductile iron are usually linearly proportional to the Brinell hardness number up to 450. However, the quantitative relationship between hardness and ultimate tensile strength is unique for ductile irons, and not universal. These specifications are the key references used to determine the heat treatment schedules, such as austempering. They also provide hints to the manufacturing process and cast iron grades in reverse engineering. The International Organization for Standardization (ISO) lists the property specifications for spheroidal graphite or nodular graphite cast iron in ISO 1083-2004 (ISO, 2004). Like SAE specifica- tions, ISO standards specify the respective (micro)structure and the hardness value for each grade. Generally, European ductile iron contains relatively less silicon (around 2.1%) than American ductile iron (ranging from 2.2 to 2.6%).
European standards are also usually more specific regarding Charpy impact values. These subtle differences make cross-referencing between different material specifications challenging in reverse engineering.
The rest of this section will focus on the three most commonly used ana- lytical techniques for microstructure identification and analysis in reverse engineering: light microscopy, scanning electron microscopy, and transmis- sion electron microscopy.
Temperature
Austenitization
Time Austempering tempering
Ms, Martensite start temperature A1, Austenitization transformation
temperature
Air cool Quench
Heat
FIgurE 5.9
Schematic of a typical austempering process.