Transition Temperature Approach to Fracture Control

The ferritic steels (carbon, low alloy, and 400 series stainless steels) all undergo a ductile-to-brittle transition as temperature is lowered. Each of these steels has a ductile-to-brittle transition temperature range. Above their transition temperature range these steels are tough; at and below the transition temperature range, they can fracture in a brittle manner.

Toughness is the ability of a material to absorb energy by yielding (plastic deformation) prior to failure. Toughness depends on a material’s ductility and strength. Toughness therefore indicates the material’s ability to resist brittle fracture.

One measure of toughness is the area under a tension stress-strain curve taken to failure. However, the standard method for pressure vessel applications is an impact test, which measures the energy to fracture of a specimen under very high strain rates (sudden impact).

Today the most widely used test for establishing the toughness of low strength steels is the Charpy V-Notch (CVN) impact test. This test has been chosen from several available because it correlates well with numerous failures, including World War II ship fractures. The CVN test is used to make sure the transition temperature of the steel is below the minimum loading temperature of the vessel. This is known as the transition temperature approach.

Figure 500-7 illustrates CVN impact test results for a carbon steel. The CVN transition temperature is defined as the minimum temperature above which the material requires more than some specified energy to break.

Illustration of Typical CVN Impact Test Data

The energy required to establish transition temperature increases with increasing steel strength. Other definitions of transition temperature, such as those based on fracture appearance, are not widely used in codes and specifications because of difficulties in interpretation. Note that the notch toughness of steels shows a temperature transition no matter what test method is used. The actual transition temperature measured will vary somewhat, depending on the test method employed.

The CVN transition temperature approach was developed empirically after World War II from analysis of ship failures. Samples from over 100 structural failures in merchant ships were studied and statistically analyzed. Fracture initiation in those steels was found to be difficult above a transition temperature corresponding to a CVN impact energy of 10 ft-lb. Crack propagation was found to be difficult above a temperature corresponding to 15 to 25 ft-lb. From these findings, a 15 ft-lb CVN requirement at the minimum loading temperatures became a widely used fracture criterion.

In the CVN impact test, notched bars are hit with a swinging pendulum. Specimens are broken over a range of test temperatures and the energy to break the specimen is recorded as a function of test temperature. CVN impact test results are in units of ft-lb (English units). (See ASTM A 370 for more details on CVN testing.) With the development of the fracture mechanics approach, it became apparent that

CVN requirements to establish transition temperature were dependent on material yield strength and thickness. Energy requirements to establish the transition temperature increase with yield strength and thickness.

ASME Code, Section VIII, Division 1, requirements for CVN energy for pressure vessel steels are given graphically in Figure UG-84.1 of the Code. Steels may be exempted from tests if they meet requirements shown in Figure UCS-66 of this Code, as discussed later in this chapter.

ASME Code, Section VIII, Division 2, requirements for CVN energy for pressure vessel steels are given in Code Table AM-211.1. The current Division 2 requirements are less conservative than the Division 1 requirements and do not take into account the need for higher CVN energy with increasing thickness. Steels may be exempted from tests if they meet requirements shown in Paragraph AM-218 of this Code, also discussed later in this chapter.

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