3.6. 2 Mechanical Properties

Mechanical properties for forging alloys, like physical properties, are listed in standard reference sources. In some cases they are not affected by subsequent manufacturing operations, and can be used with reasonable confidence to predict real world performance. In other cases, mechanical properties are altered by subsequent processes, in varying amounts and with varying degrees of predictability in the end product. Variations are caused by factors such as:

  • Forging temperature
    Forging reduction (deformation) which, in turn, affects grain size
  • Heat treatment.

In some cases an experienced forging engineer can predict properties, such as yield strength, in critical areas of the forging with reasonable accuracy. Predictability is enhanced by two characteristics of forgings.

  1. Forgings are fully dense and not subjected to discontinuities, such as porosity in castings.
  2. Forging alloys are homogeneous, and not subject to variations in composition, such as orientation of reinforcing fibers in composites.
  3. The essential mechanical properties of forging alloys and the effect of processing are summarized in Table 3-3.

Table 3.3 Effects of Processing on Mechanical Properties
Property A: Hot Forged Without Heat Treatment B: Hot, Warm or Cold Forged and Heat Treated C: Warm or Cold Forged Without Heat Treatment
Tensile Strength
Yield Strength
Hardness
Elongation
Reduction in Area
Modulus of Elasticity
Poisson's Ratio
Impact Toughness

Fracture Toughness

 
Fatigue Strength
Subject to variations due to variations in cooling rates. Varies within the forging with section size, heat treatment and material hardenability.. Warm forging, varies due to variations in cooling rate.
No variation No variation. No variation
Will vary with variations in cooling rate and forging temperature. Can be enhanced by control of grain flow. Affected by grain flow and controlled by heat treatment.

Cold forging, will vary with amount of grain flow.

Warm forging, will vary with variations in cooling rate.

 

The variations in properties due to processing have various implications on design, depending on the critical requirements of the application. The implications are of special interest in the preliminary design stage when computerized engineering tools such as finite element analysis or modal analysis are used. Several classes of application are summarized in Table 3-4.

Table 3-4 Performance Predictability for Selected Design Criteria
Design Criteria Critical Properties for
Modeling		 Forging Process and Heat

Treatment

Deflection or vibration response at stress levels below yield strength Density, modulus of elasticity and Poisson's Ratio Performance is predictable for all groups, but yield strength may be difficult to predict for Groups A and C.
Static loads developing stresses below yield strength Modulus of elasticity, Poisson's Ratio, yield strength Minimum performance is predictable for Group A. Performance is usually better for groups B and C, but properties are more difficult to predict an may vary within the part.
Static loads developing stresses in the plastic deformation range below fracture Modulus of elasticity, Poisson’s Ratio, yield strength and plastic stress-strain data.
Fatigue caused by cyclic loading Fatigue Strength
Impact loads causing gross plastic deformation Impact strength Difficult to model regardless of forging process and heat treatment unless input loads can be predicted.

 

The mechanical properties of a forging are best optimized for each application when the product design engineer identifies to the forging engineer those areas of the product where performance is critical, and the properties required. The earlier this occurs in the design process, the better able is the forging engineer to tailor the design and process to achieve optimum performance.

In those cases where performance must be optimized beyond the capabilities of computer aided engineering with available material properties data, particularly where material must be minimized due to weight limitations or material cost, the concurrent engineering team may perform an iterative process of test-and alter by:

  • Designing and evaluating the product as closely as possible, with maximum anticipated values for properties, using computer aided engineering techniques
  • Producing sample forgings and finishing as required
  • Evaluating by test
  • Modifying the design and re-testing as necessary

In this type of development program, the fewer the number of iterations, the lower the cost and the shorter the development cycle. It is critical to remember that products made from forgings are formed in hard dies, which are the negative of the finished product. Material is added to the product by removing metal from the die, and removed from the product by adding material to the die. It is therefore more practical to add material to the product than remove it.

The recommended practice, when close optimization is required, is to underdesign slightly on the initial design, and subsequently add metal to critical sections. The alternative, to overdesign and subsequently remove metal from the part requires adding metal to or resinking the die, and should be discouraged.

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