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Identifying the Cause of Tool and Die Failure

Steels used for tools and dies differ from most other steels in several aspects. First they are used in the manufacture of other products by a variety of forming processes. Second, tools and dies are generally used at higher hardnesses than most other steel products; 58 to 68 Rockwell C is a typical range. Dies for plastic molding or hot working are usually used at lower hardnesses, typically from 30 to 55 Rockwell C.

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Fractography

Fracture Modes

Transgranular: Cracking across grains without preference for grain boundaries

Intergranular: Cracking between grains, the crack propagates in the grain boundaries

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Failures of Tools and Dies

FAILURE MECHANISMS in tool and die materials that are very important to nearly all manufacturing processes are discussed in this article. A wide variety of tool steel compositions are used. Properties and selection of tool steels are described in the Section “Tool Materials” in Volume 3 of the 9th Edition of Metals Handbook; microstructures and metallographic techniques for tool steels are detailed in the article “Tool Steels” in Volume 9 of the 9th Edition of Metals Handbook. This article is primarily devoted to failures of tool steels used in cold-working and hot-working applications.

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Failures of Locomotive Axles

FAILURES OF LOCOMOTIVE AXLES

caused by overheated traction-motor support bearings are discussed in this article. These failures are of interest because the analysis shows an example of what can be done when the fracture face and origin are destroyed during the failure incident. In most failure analyses of broken components, it is generally assumed that conclusive results cannot be obtained if the fracture and the fracture origin cannot be identified and examined. In many failures this is true. However, the failures described in this article possess some unique characteristics that permit successful analysis despite the lack of a preserved fracture face and origin.

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Failure Analysis – Service Problems

Intergranular SCC in 4340 Alloy Steel

Aqueous saturated picric acid, with 1% HCl and a wetting agent, Nacconol
90G, at 80°C revealed the prior-austenite grain boundaries. The hardness was too high, 35-38 HRC, making the steel sensitive to Cl-ion stress- corrosion cracking.

SCC Failures of ASTM A325 Bolts in the Piscataqua Bridge

Fracture appearance of a typical failed bolt; arrows point to the origin at the surface. There is some corrosion on the surface (3x, 6x)

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Conducting the Failure Examination

Not all failures are catastrophic. Many failures involve a gradual degradation of properties or excessive deformation or wear until the component is no longer functional. Failures due to wear or general corrosive attack usually are not spectacular failures, but account for tremendous material losses and downtime every year. Of course, early failures of the spectacular catastrophic order capture the most attention and rightly so. Nevertheless, all failures deserve the attention of the investigator because they reduce production efficiency, waste critical materials, and increase costs.

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Conducting the Failure Analysis

Non-Fracture Failures

  • Ductile Deformation
  • Creep
  • Distortion
  • Wear
  • General Corrosion
  • Leakage, e.g., Selective Leaching, Pitting, Crevice Corrosion

Causes of Failure

  • Poor Design
  • Imperfections in Materials
  • Imperfections in Manufacture/Fabrication
  • Overloading/Service Abuse
  • Improper Maintenance or Repair
  • Environmental Effects
  • Combinations of the above

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