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IN-PROCESS OR SERVICE FAILURES of forgings may occur for a variety of reasons. The starting material may be of insufficient quality to be adequately formed without cracking, or the forging process may introduce various types of discontinuities that cause failure during services.

For example, well-known forging-related dis-continuities include:

Parting-line grain flow

Inclusions

Forging discontinuities are discussed in more detail in the texts on forging (Ref 1–4).

This article describes six case studies of failures with steel forgings (summarized in Table 1). The case studies illustrate difficulties encountered in either cold forging or hot forg-ing in terms of preforge factors and/or dis-continuities generated by the forging process.

Tables 2 and 3 summarize these factors for cold and hot forging, respectively. Supporting

topics that are discussed in the case studies include:

Validity checks for buster and blocker design

Lubrication and wear

Mechanical surface phenomenon

Forging process design

Forging tolerances

As case studies were being selected, each of the aforementioned supporting topics was reviewed for any impact that particular study had on the case being examined. It is a well-known fact that forging solutions have several possible avenues to follow. There is no unique theory in plasticity that leads to the solution.

Most of the work reported here was performed using the minimum amount of energy to create the particular product. Factors unrelated to the deformation process, such as chemistry, micro-structure, phase, grain size, segregation, and prior strain history, are not addressed here.

Instead, factors directly related to the deforma-tion process itself are presented in this abbre-viated discussion.

Wear, plastic deformation processes, and laws of friction are introduced as a group of

Table 1 Failure analysis of steel forgings and components

Case study Defect Solution

Crankshaft underfill Unable to fill crankshaft flanges with existing press capacity

Introduce creep stages for last increment of displacements

Tube bending Unable to control exterior wall thinning and interior wall thickening

Introduce induction heating and cooling to limit the heated axial tube length prior to making the bend Spade bit Unable to achieve center web thickness at programmed

force and sufficient flow to wings

Adjust the die angle to create more shear stress, enabling full flow to the wings

Trim tear Forge material tore at trimline when forging was trimmed immediately following finish forging

Introduce a delay time after forge and prior to trim, allowing the forge material to cool and gain strength Upset forging Cracking at circumferential bulge after upset Re-examine the strain and strain rate and process map

for stable flow Flow-through laps

and avoidance

Material foldover at tops of rib and flange intersections and cases of material flow under previously filled flanges

Replace the input piece with a newly designed preform piece, following the design procedures given in this work

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Failure Analysis of Heat Treated Steel Components

L.C.F. Canale, R.A. Mesquita, and G.E. Totten, editors, p 133-149 DOI: 10.1361/faht2008p133

Copyright © 2008 ASM International®

All rights reserved.

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subjects that have been considered in the case studies. Added factors that were evaluated in the case studies are:

Crankshaft underfill: induction coil inside diameter and stock diameter, equivalent current depth and subsequent time for con-duction to reach a uniform stock tempera-ture, total heating time for scale control, transfer time to press, and forging force applied

Tube bending: precise heat input, control of temperature, and heated axial length of tube

Spade bit: direction of forging relative to part shape and assessment of shear effect in extended wings

Trim tear: trimmer tool tolerances, part temperature, and process time

Upset forge: principal strains and equivalent plastic strain

Flow through: strains going from round or flattened piece to finish, and assessing the need for a more generalized shape for input to finish die. Preform design—streamline

shape (not the same for aluminum-, mag-nesium-, steel-, titanium-, or nickel-base alloys)

Lubrication: select one that provides the lowest coefficient of friction and other acceptable properties

Forge process: total process for entire manufacturing train, including heat treat-ment and product testing

Forge checking: fixture check for critical dimensions

Forge tolerances: component to fit the cus-tomer’s assembly

Simulation of process: verify that laws of plasticity are met

Forging Process Design

Forging process design requires the applica-tion of integrated engineering principles that bring together factors such as:

Relationship between the important sub-system of a deformation sub-system (Fig. 1) Table 2 Factors in analysis of cold forging failures

Preforge factors

1. Raw material—chemistry, microstructure, mechanical properties, size, surface finish, and cleanliness 2. Shape sequencing—general nature of shape to be created; strain, strain rate, and load requirements

3. Forging—equilibrium forging temperature, strain and strain rate, workpiece volume control, forge equipment, loading and transfer devices, lubrication, parts collection, inspection, and annealing

4. Trimming

Causes of defects during cold forging

1. Cracking—Three factors combine to produce cracks: stress from thermal expansion and contraction, hydrogen, and a susceptible microstructure.

2. Product underfill—poor flow, sufficient volume, and proper distribution

3. Unbalanced forces—laps/lap fillin, nonhomogeneous strain, strain rate, nonuniform microstructure, and work hardening

a. Seams—external and internal—on or within a metal surface, an unwelded fold or lap that appears as a crack usually resulting from a discontinuity

b. Inclusions—raw material; internal and external substance that is foreign and insoluble to the matrix; particles of a foreign material in metallic matrix. Particles are usually compounds, such as oxides, sulfides, or silicates but may be of any substance that is foreign and insoluble to the matrix.

c. Tears—occur when the equivalent plastic strain exceeds the capability of the material

d. Entrapped scale—forged in contamination consisting primarily of oxides but can include other products left on metals

4. Strain hardening—increase in hardness and strength of metals caused by plastic deformation at temperatures below the recrystallization range; also known as work hardening

5. Flow through/push through—condition at which excessive material is provided in the preform in error, such that as elements of the shape are completely filled, such as flanges or rails, the central material continues to displace outward underneath the filled flanges 6. Porosity/voids—small openings, interstices, or channels within a consolidated solid mass or agglomerate usually larger than atomic

or molecular dimensions

7. Segregation—In the casting process, the solidifying front moves away from the surface of the casting as a plane front, and lower-melting-point constituents in the solidifying alloy are driven toward the center. This is called normal segregation.

8. Internal shearing—This effect can occur when material displacements cause excessive sliding of adjacent volumes of material.

9. Surface impurities—any foreign substance deposited on the part unintentionally

10. Grain size structure—The number of grains per unit volume and the phase of the material dictate the forging response.

11. Flakes, blisters—These flaws typically result from the raw material or other processing steps but may show up when materials are forged.

12. Residual stresses/distortion—Most materials (especially steels) will have residual stresses after cold forging; distortion occurs when the stresses are not symmetrical.

13. Lubrication—dies and workpiece—viscosity and flow, hydrodynamics of lubrication, friction, heat generation and power losses, coefficients of friction

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Interdependence of forging process para-meters (Fig. 2)

Forging process design task overview (Fig. 3)

Relationship between process and machine variables (Fig. 4)

Characteristics of forging machines (Table 4)

Workability modeling (process maps show-ing zones of stable flow) of workpiece

behavior to achieve stable deformation at a specified rate and proper evolution of microstructures and properties

Forging Tolerances

The need for verification of the nominal dimensions and application of forging toler-ances is important for quality assurance. Toler-ances are required on forged products to allow for practical variations in die preparation, tem-perature effects during forging, equipment, and distortion during and after heat treatment. Forg-ing tolerance review is a basic requirement to ensure that the part meets the multitude of design features and tolerances. A listing of the more important forging tolerances includes:

Dimensional—length, width, center-to-center, and external-internal

Die wear—generally approximately 0.102 mm (0.004 in.)/surface

Die closure—thickness of approximately 0.813 to 6.35 mm (0.032 to 0.250 in.) as a function of plan form area at trimline Table 3 Factors in analysis of hot forging failures

Preforge factors

1. Raw material—chemistry, microstructure, mechanical properties, size, surface finish, and cleanliness 2. Shape sequencing—shape nature, temperature, strain and strain rate, upset tooling, fuller (roll), open-die tooling

3. Hot forging—temperature, strain and strain rate, forge center cell, loading and transfer device, lubrication, parts collection, and inspection 4. Trimming—trimmer unit and capacity, flash removal, temperature trace of product flashline

Causes of defects during hot forging

1. Cracking—occurs when the imposed equivalent plastic strain exceeds the material capability at the temperature of operation—surface (hot tears), cooling (centerline cracking)

2. Product underfill—underachieved thickness goal, inadequate material displacements, poor 3-D flow, inability of input shape to subsequent follow-on dies to satisfy local volume requirements, control of centroid path of newly created shapes

3. Unbalanced forces—laps/lap fillin, nonhomogeneous strain, strain rate, nonuniform and continuous microstructure

a. Seams—external and internal—on or within a metal surface, an unwelded fold or lap that appears as a crack usually resulting from a discontinuity

b. Inclusions—raw material; internal and external substance that is foreign and insoluble to the matrix; particles of a foreign material in metallic matrix. The particles are usually compounds, such as oxides, sulfides, or silicates but may be of any substance that is foreign and insoluble to the matrix.

c. Hot tears—occur when the equivalent plastic strain exceeds the capability of the material at the temperature of operation d. Entrapped scale—forged-in contamination consisting primarily of oxides but can include other products left on metals 4. Flow through/push through—condition at which excessive material is provided in the preform in error, such that as elements of the

shape are completely filled, such as flanges or rails, the central material continues to displace outward underneath the filled flanges 5. Porosity and voids—small openings, interstices, or channels within a consolidated solid mass or agglomerate usually larger than atomic

or molecular dimensions

6. Segregation—In the casting process, the solidifying front moves away from the surface of the casting as a plane front, and lower-melting-point constituents in the solidifying alloy are driven toward the center. This is called normal segregation.

7. Internal shearing—This effect can occur when material displacements cause excessive sliding of adjacent volumes of material.

8. Surface impurities—any foreign substance deposited on the part

9. Grain size structure—The number of grains per unit volume and the phase of the material dictate the forging response.

10. Flakes/blisters—These flaws typically result from the raw material or other processing steps but may show up when materials are forged.

11. Residual stresses/distortion—Most steel forgings will have inherently residual stresses and distortion due to cold straightening or following quenching.

12. Lubrication—dies and workpiece—viscosity and flow, hydrodynamics of lubrication, friction, heat generation and power losses, coefficients of friction

Equipment Material

system

Control system Constitutive

Equation

Workability Control

System

Fig. 1 Relationship between important subsystems of a de-formation system. Source: Ref 5

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Match—alignment of the top and bottom dies

Radii—strong influence on material dis-placements

Flash extension

Straightness—taken as a separate feature and then assessing its effect on the remainder of other tolerances

Data on Billet material

Ram velocity Strain rate

Billet/Forging Geometry, Volume and thickness

Die temperature, cooling

Interface lubrication

Contact time under pressure

Temperature distribution in

forging

• Metal flow

• Forging load

• Forging energy

Friction Conditions and

coefficient Flow stress/

forgeability Data on

Billet material

Ram velocity Strain rate

Billet/Forging Geometry, Volume and thickness

Die temperature, cooling

Interface lubrication

Contact time under pressure

Temperature distribution in

forging

• Metal flow

• Forging load

• Forging energy

Friction Conditions and

coefficient Flow stress/

forgeability

Fig. 2 Interdependence of forging process parameters

Fig. 3 Forging process design task overview

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Draft angle

Datum plane location for three-plane (x, y, x) setup

Alternative machined tooling points

Finish allowance between forging and machined part

Tolerance review is conducted in various ways, and in the past, numerous forgings have been rejected and held for material review until some decision could be reached regarding their disposition for rejection or alternative repair.

Even though this has typically been from review of the part drawings, another useful way to assess a completed forging is a fixture check. A uniquely designed fixture in conjunction with a dimensional inspector sets the forging into fixed tooling point locations and proceeds with the check “go” or “no-go,” determining whether the part will or will not serve its function in the assembly. In many cases, special fixtures and

gages can confirm the accuracy of dimensions that are critical to the function of the component dimension. Large forgings are good candidates for fixture checking.

Wear and Lubrication

Surface interactions of two materials are influenced by small regions where contact is made at the atomic level. The real area of contact is determined by elastic and plastic deformation under consideration of loading. Lubrication reduces friction by introducing a viscous and low-shear-strength layer at junctions. Surface interactions can lead to wear, or the removal of material as a result of mechanical action.

Wear types include:

Adhesion wear: particle transfers (pulled off) from one and adheres to the other

Abrasive wear: a hard, rough surface plows grooves into the softer one

Fig. 4 Forging equipment characteristics; relationship between process and machine variables

Table 4 Characteristics of forging presses

Equipment type Deformation rate Temperature loss Consistency Production rate

Hydraulic press Slow High Very good Low

Mechanical press Slow to medium Moderate Good Moderate to high

Screw press Moderate to high Moderate Fair to good Moderate to high

Hammer High Low Fair to good Moderate

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Corrosive wear: mechanical action removes a protective layer from a surface and exposes it to corrosive attack

Surface fatigue wear: spalling occurs after the formation of surface or subsurface cracks

Volume wear: proportional to the load and distance traveled and inversely proportional to the material hardness

During the early 1950s, the importance of proper lubrication was recognized on the shop floor. If an inexperienced oiler inadequately applied lubricant (in spray or paste form), then forging problems could occur even for an acceptable preliminary workpiece (preform).

Alternatively, a questionable preform for the first closed impression die would be proven acceptable if an experienced oiler knew where the lubricant should be applied over the die impression and also when the die impression needed additional heavy lubricant in given die locations that appeared difficult to fill.

These anecdotes vanished quickly as more science replaced art in forging. Presently, there are numerous ways that lubricants are used in the forging industry. Wrapping the workpiece dur-ing heatdur-ing is an approach to prevent the for-mation of scale in the case of steel or thin metal sheets or cloths with impregnated graphite, in addition to the automatic spraying of lubricants.

Lubricants play an important part in forging by minimizing the load required for maximizing material flow, protecting the die surface finish (critical for a specific lubricant), and assisting the entire forging process.

Lubricant performance factors include:

Adequate lubricity

Stability in gas-fired and electric furnaces

Protect stock against atmospheric conta-minants

Provide good surface finish

Act as a release agent

No buildup in die cavity

Ease of application and removal

Conform to Environmental Protection Agency (EPA) and Occupational Safety and Health Administration (OSHA) require-ments

Acceptable cost

Compatible with die materials

Graphite products for forging lubrication are:

GPC—for hot and warm forging

Die lubricants—GP series

GP 100—low dilution ratios and spray application

LS—oil and water

Precoat workpiece—contains graphite as a lubricant pigment

Adhesion colloids are reliable for high pressure and temperature. Types include:

Colloidal—dispersions

Delta forge lubricants—for hammer, press, and upsetters

Deltaglaze—protective lubricants for billets applicable to steel

Case Studies

Case Study 1: Crankshaft Underfill. There are several large steel forging components, such as ship crankshafts and airplane landing gear, being manufactured successfully in the United States and throughout the world today (2008).

Crankshaft forgings in the weight range of 2268 to 4536 kg (5000 to 10,000 lb) are products made by a forging process creating a pair of flanges and a pinion shaft diameter at one time.

The inboard and outboard flanges along with the pinion diameter become integral parts of the main shaft diameter.

The forging operation creates one set of flanges by means of a working stroke in line with the major shaft diameter, while a 90^off-set load forges the pinion shaft between the flanges.

These operations are generally performed fol-lowing one local heating of the starting bar diameter for forging a set of crankshaft throws, including the two flanges and an offset pin dia-meter. The forging process is repeated until all of the flanges plus the offset pinion diameter are created along the major diameter of the crank-shaft. The nature of the ready-for-assembly fin-ish forging design for the incrementally forged crankshafts includes locations where material is provided for machining along with selected as-forged surfaces.

During the forging of the flanges, there had been cases of small amounts of underfill at the flange extremities, as shown in Fig. 5. That extent of underfill has caused the entire com-ponent to be rejected.

A test run was planned to measure material displacements while the flanges were being forged at the prior selected process variables of strain, strain rate, temperature of workpiece and dies, and forging force exerted. The conclusion 138 / Failure Analysis of Heat Treated Steel Components

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reached was that since the workpiece material temperature at the end of the press working stroke was still in the hot working range, an extended length of time with the maximum force applied would be helpful to displace the rela-tively small amount of missing material into the remote regions of the flange dies. The thought was that allowing material to creep would aid in the final filling of the die cavities.

Creep is an example of viscous flow and is defined as continuing flow at constant stress. At characteristic stresses, the creep strain reaches a steady state in which the rate of straining is constant. This is called the steady-state creep

Creep is an example of viscous flow and is defined as continuing flow at constant stress. At characteristic stresses, the creep strain reaches a steady state in which the rate of straining is constant. This is called the steady-state creep

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