Piping Material with Hardness and Toughness of the Material's Ability

Continuing the Piping Material with Hardness and Toughness of the material's ability and Piping & Fabrication will give the detail information of it.

Hardness

This is a measure of the material’s ability to resist deformation, usually determined by a standardized test where the surface resistance to indentation is measured. The most common hardness tests are defined by the indentor type and size, and the amount of load applied. The hardness numbers constitute a nondimensioned, arbitrary scale, with increasing numbers representing harder surfaces. The two most common hardness test methods are Brinell Hardness and Rockwell Hardness, with each representing a standardized test machine with its own unique hardness scales. Hardness loosely correlates with ultimate tensile strength in metals (Fig. A3.5). Approximate hardness conversion numbers for a variety of material types, including steels, can be found in ASTM Specification E140.

FIGURE A3.4 Offset method of determining yield strength

Toughness
Sudden fracture, exhibiting little ductility in the vicinity of the break, occurs in certain metals when load is rapidly applied. The capability of a material to resist such a brittle fracture is a measure of its toughness. Highly ductile materials (those possessing an FCC lattice, for example) exhibit considerable toughness across a full range of temperatures. Other materials, such as BCC-based carbon steels, possess a level of toughness that is dependent on the metal temperature when the load is applied. In these metals, a transition from brittle to ductile behavior occurs over a narrow range of temperatures.

 

FIGURE A3.5 Conversion chart for Brinell and Rockwell
hardness numbers, giving corresponding tensile strength for
steel. Based on hardness conversion table

The two most common methods used to measure metal toughness are the Charpy Impact test, defined in ASTM specification E 23, and the Drop-Weight test, defined in ASTM E 208. The Charpy test employs a small machined specimen with a machined notch that is struck by a pendulum weight (Fig. A3.6). The energy loss to the pendulum as it passes through and breaks the specimen (Fig. A3.7), measured in kilojoules or ft _ lb of force, is a measure of the toughness of the specimen. Typical impact behavior versus test temperature is shown in Fig. A3.8.

The Drop-Weight test is similar in principle but employs a larger specimen with a brittle, notched weld bead used as the crack starter (Fig. A3.9). A weight is dropped from a height onto the specimen, which had been cooled or heated to the desired test temperature. The test determines the nil-ductility transition temperature (NDTT), defined as the specimen temperature when, upon striking, the crack propagates across the entire specimen width. Propagates across the entire specimen width.

 

FIGURE A3.6 Charpy (simple beam) subsize (Type A) impact test specimens

The Charpy brittle transition temperature (sometimes called the Charpy fix temperature) and the Drop Weight NDTT are both important design considerations for those materials that can exhibit poor toughness and that may operate in lower temperature regimes. In pressure vessel and piping design codes, limits are placed on material minimum use temperature based on adding an increment of margin over and above the Charpy fix or NDTT. Operating at or above this elevated temperature is then usually sufficient to avoid brittle, catastrophic failure, as for example is the case when at a temperature on the ‘‘upper shelf’’ of the Charpy Vnotch toughness-versus-temperature curve.

 

FIGURE A3.7 Charpy V-notch specimen placement during strike by testing anvil