Technological advances have made it possible to reduce the material design factor in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. Today solid-state circuitry is the only power source used in welding equipment. The use of solid-state circuitry has brought a great deal of stability to the electrical characteristics of the arc, and this in turn has resulted in weldments with considerably fewer imperfections. The US government sponsored a study to determine the causes of the failures and to evaluate the metallurgical and other factors that contributed to the fractures. This project studied samples from more than 100 ships, in which serious failures had occurred. This and other concurrent studies around the world led to an understanding of the role of notch toughness in preventing failures in pressure boundary components. The conservativeness of Section I and Section VIII, Division 1, continues to be significant. The change from 4 to 3.5 in the material design factor will not compromise safety. The denominator for determining an allowable stress based on yield strength remains the same, 1.5. The probability of failure of a pressure-containing component due to an overstress condition is small. The change from 4 to 3.5 has had very little impact on that conservative material design factor.
A recent change in the ASME Boiler and Pressure Vessel Code has lowered the material design factor that is used in calculating allowable stress values under the Code’s Section I (power boilers) and Section VIII Division 1 (pressure vessels).
The new figure, found in the 1999 Addenda to the 1998 Edition of the Code issued on July 1, 1999, contains allowable stress values based on a material design factor of 3.5, rather than 4 (the figure in use for more than a half-century). The number appears in the denominator of one of the equations used to establish allowable stress values. Advances over the past 55 years in the technologies of steel making and nondestructive examination and in the ability to assess fracture toughness make this change in the material design factor long overdue.
The allowable stress values are determined by dividing the tensile strength of a material by a denominator, which is a material design factor. This procedure for developing allowable stresses has been essentially unchanged since the first edition of the ASME Boiler and Pressure Vessel Code in 1914. The formula used in the 1914 edition for determining the maximum allowable working pressure contained an element that was identified as a factor of safety, or FS. The FS, multiplied by the radius of the vessel, was in the denominator of the equation; this was divided into the product of the room temperature ultimate tensile strength (TS), the efficiency of the longitudinal joint or the ligaments between tube holes (E), and the minimum thickness of the shell plate (t). Maximum Allowable Working Pressure — (TS) x (t) x (E)/(R) x (FS).
The FS in the denominator was 5 in the 1914 edition of the code. This was later carried over as the basis for establishing allowable stresses and became in essence a material design factor. This change in terminology was used to avoid the misunderstanding that this is the sole safety factor when designing a pressure part. The value of 5 remained in force until 1944, when World War II gave rise to an intense need to conserve materials.
Domenic A. Canonico is the Chair of the Main Committee for the ASME Boiler and Pressure Vessel Code. He is also the 1999 recipient of ASME’s Melvin R. Green Codes and Standards Medal.
A special committee was asked to review a request to reduce the material design factor from 5 to 4. This committee favored the change, and justified it by citing “great improvements in the art of welding.” This review resulted in a code case (968) stating that such a material design factor could be applied providing the main joints were seamless or fusion-welded. This was a dramatic change of 20 percent in allowable stress, with little technical basis other than those “great improvements in the art of welding.” Around this time, the hydrostatic test pressure was also changed from 2 to 1.5.
Most welding processes that are taken for granted today were in their infancy when the material design factor was lowered in 1944. The reasons for the change were more emotional, or at any rate war driven, than technologically sound. After the war, the material design factor was returned to 5 until 1951. In 1950, it was decided, based on the excellent service provided by pressure equipment designed with a material design factor of 4, that the factor should be permanently reduced to 4. This was done in the August 8, 1951, Addenda of the published code.
The material design factor of 4 had been in effect for decades when it was challenged by the chairman of Section VIII in 1995. As a result, the Pressure Vessel Research Committee reviewed the advisability of a change. The committee concluded in 1996 that it was technically sound and safe to reduce the material design factor from 4 to 3.5. The committee looked at a number of items, including the requirements of foreign codes. But one need look no further than the evolution of the materials and the technologies that affect the quality and safety of a structure to find a rational justification for the change.
Advances in the technology of welding since 1944 have been outstanding. Fusion welding for pressure-retaining components was not accepted as a fabrication practice by the Boiler and Pressure Vessel Code until the 1931 edition. Thirteen years later, the committee felt comfortable reducing the material design factor by 20 percent because of “improvements in welding.” Most of the welding processes that are accepted as state-of-the-art today were, in 1944, either in their infancy (gas metal arc, gas tungsten arc, and submerged processes) or had not been developed (low hydrogen electrodes, flux core process, electro-slag process, electron beam process, and laser welding process).
Advances in the electronics of power supplies have also been impressive. Today solid-state circuitry is the only power source used in welding equipment. The use of solid-state circuitry has brought a great deal of stability to the electrical characteristics of the arc, and this in turn has resulted in weldments with considerably fewer imperfections. In 1944, vacuum tubes were the state-of-the-art for electronic circuitry; the semiconductor was not invented until the mid-1950s.
The fact that the base metal is exposed to high thermal excursions over short distances during welding was recognized early, and the need to relieve the weldment of residual stresses brought about by these thermal differentials became a requirement of the code. The concept of fracture toughness was a laboratory curiosity in 1944 and the idea of postweld heat treating, as opposed to heating for the relief of residual stress, had not been conceived. As late as the 1962 edition of the ASME Boiler and Pressure Vessel Code, neither Section I nor Section VIII contained a reference to postweld treating or preheating. Today it is mandatory in Sections I, III (nuclear components), and VIII that all weldments be postweld heat treated (PWHT). Exceptions to the mandatory requirements for PWHT impose specific rules that must be satisfied.
As hydrogen-induced delayed cracking moved out of the laboratory and into the commercial arena, the importance of preheating came to be recognized. Though preheating is not mandatory today in Sections I, III, or VIII, suggestions for preheating are given in nonmandatory appendices to those code books.
Nondestructive examination, which did not come into its own until after World War II, today assures that a product has the integrity required for safe operation. When the material design factor was reduced in 1944, the only viable nondestructive examination technique was radiography, which at that time was in its early stages of development.
During the 1920s, radiography was used to examine materials. It was not until welding became commonplace for the construction of pressure equipment in the 1930s that the radiographic examination of weldments became an acceptable method for the volumetric examination of thick sections. The 1935 edition of the Boiler and Pressure Vessel Code limited welded construction to a maximum thickness of 4 inches, because that was considered the limit that X-rays could penetrate a steel shell and provide meaningful results. It wasn’t until the late 1930s that radiography was sufficiently advanced so imperfections such as slag inclusions and lack of penetration could be identified in the X-ray film. Moreover, the use of high-energy radiation sources for radiography was still being developed in laboratories throughout the world in the 1940s.
Radiography typically has a resolution of about 2 percent; an imperfection of about 3/32-inch may not be detected by radiography in a 5-inch section size. An imperfection considerably smaller than that can be easily detected by ultrasonic testing, but the ultrasonic process did not realize its true potential until after the angle beam probe was developed in 1947. Today the ultrasonic process is used to determine not only the quality of a material product form (for example, through-the-thickness quality of plate and forgings) and the integrity of a weldment, but also to determine the thickness of a component.
Another nondestructive examination procedure that can detect extremely small imperfections is the eddy-current process. This process is viable for any electrically conductive material. The process was under development as early as the 1930s, but was not commercially available for use until well into the 1950s.
Research over the past five and a half decades has brought new strength and reliability to materials.
Failures among tankers and Liberty ships during World War II led to a greater understanding of the concept of fracture toughness, as well as how to improve it. Fracture toughness is a measure of a material’s resistance to fast fractures that undergo rapid propagation with little or no evidence of detectable plastic flow.
Such brittle fractures most commonly occur in dynamically loaded ferritic materials with body-centered-cubic, or bcc, crystal structure in which an imperfection serves as a stress concentrator. The resistance to such a failure in the presence of a stress concentrator is referred to as “notch toughness,” from the notched bar impact tests, such as the Charpy V-Notch test, which measure this property.
In general, bcc materials tend to become more susceptible to brittle fracture as the temperature drops.
At the start of World War II, the design of large welded ferritic structures was based on tensile properties. The failure of several large structures, especially ships in the United States merchant fleet, at stress levels considerably below their yield strength made it clear that properties besides tensile strength must be considered. Nearly 80 ships broke in two, and almost 1,000 were found to have deck plates with long brittle fractures.
The U.S. government sponsored a study to determine the causes of the failures and to evaluate the metallurgical and other factors that contributed to the fractures.
This project studied samples from more than 100 ships in which serious failures had occurred. The ship failures occurred at temperatures that ranged from about 28°F to about 35°F. It also found that, while failures also occurred in riveted as well as welded ships, many failures stopped at the discontinuity of the riveted joints. A welded joint, on the other hand, could provide a continuous path and facilitate cracking.
It was determined early that failures were generally of a brittle nature. The study also showed that higher levels of carbon, phosphorous, molybdenum, and arsenic in the alloy increased the transition temperature, while nickel, silicon, manganese, and copper decreased the transition temperature.
This and other concurrent studies around the world led to an understanding of the role of notch toughness in preventing failures in pressure boundary components. But this information was not widely known in 1944.
The drop weight test was not developed until the late 1940s, and the concept of fracture mechanics was a laboratory curiosity in 1944. The usefulness of these two tools came into focus during the birth of the peacetime use of nuclear power as a heat source to generate electricity.
Fracture mechanics provides a method to quantitatively measure a material’s ability to resist brittle fracture. The concept of applying fracture mechanics for pressure vessels, and the data that are needed to make it a viable assessment method for determining the serviceability of a pressure component, were extensively discussed in a 1967 publication by the Oak Ridge National Laboratory. This document led to the formation of the Heavy Section Steel Technology program, which brought the concept of fracture mechanics into the realm of practicality.
Fracture mechanics is used in Section XI of the ASME Boiler and Pressure Vessel Code to determine the integrity of an operating nuclear pressure vessel. Further, the concept of fracture mechanics is the basis for the recent revisions to the toughness requirements in Section VIII and serves as a major facet of the design considerations in the recently published code (Section VIII, Part 3) for the design and fabrication of pressure vessels for service at very high pressures.
Since World War II, there have been great strides in the basic understanding of the role that various elements play in the quality of steel plate. The Ship Structure Committee study provided a great deal of information about the effect of various elements, particularly their influence on notch toughness. Those data, along with other ongoing research studies, have led to great improvements in the melting and refining of steels for structural applications.
Sulfur affects the cleanliness of a steel and, in turn, its notch toughness. It both raises the transition temperature and lowers the upper shelf energy. In the late 1950s and early 1960s, it was common for steel plate to contain upward of 0.02 percent sulfur, whereas today sulfur levels of 0.002 percent are prevalent. Sulfur that is present is shape controlled through the addition of rare earth elements.
Along with carbon, phosphorous has a great effect on lowering the notch toughness of steels. Today, due to the development of the basic oxygen process, phosphorous levels in steel are frequently less than 0.01 percent.
Melting practices have also improved over the past 55 years. There have been dramatic advances in the processing of steels. The commercialization of many processes has led to considerably cleaner, and more notch-tough, materials. Among the primary steel melting processes are the electric arc furnace and the basic oxygen process. Processes that include vacuum degassing, electro-slag remelting, and argon oxygen degassing result in control of gases (less oxygen and hydrogen), low sulfur content, microcleanliness (that is, the removal of undesirable nonmetallics), better control of the change in composition and shape of inclusions, and improved mechanical properties.
Studies in the late 1940s and early 1950s showed the beneficial aspects of certain alloying elements. For instance, researchers revealed the influence of manganese in controlling the detrimental effect of sulfur, particularly during welding. As a result of a brittle failure in the high-temperature steam line at a power plant in Pennsylvania in 1942, it was learned that small additions of molybdenum to carbon steels prevented graphitization. It was determined that the power plant failure occurred as a result of the metastable iron carbide dissociating into graphite and iron after long periods of time at temperatures above 850°F. Further, it was determined that steels melted to fine-grained practice (aluminum killed) had superior notch toughness properties.
The effect of lowering the material design factor from 4 to 3.5 is to increase the allowable stress values in Tables 1A and IB of Section II. Part D has a maximum of 14.3 percent in the temperature regime where time-independent mechanical properties control these values. There have been no changes to the allowable stress values in the time-dependent temperature regime, the temperatures at which creep and stress rupture properties are dominant. If a pressure boundary component is designed for service at room temperature and, depending on the alloy, perhaps up to 500°F, a nearly 14 percent increase in allowable stress will persist. For many carbon and low-alloy steels, the 14 percent advantage begins to decrease at about 500°F, and for carbon steels it is zero at 750°F.
For SA-516 Grade 70, the effect of the change in the material design factor is to provide a 7.4 percent increase in allowable stress at 650°F, a typical design temperature for this grade of carbon steel. At that same temperature, the increase in allowable stress for SA-299 is only 5.3 percent. A similar effect is observed for SA-213 T12 and T22. The differences between the 1998 and 1999 Addenda allowable stresses for these two alloys are 3.6 percent and zero respectively at 900°F, a temperature in the range where the higher price for these alloys becomes cost effective.
Protecting Public Safety
Everyone is concerned with protecting the safety of the public. That was the objective in 1911 when the ASME Council appointed a committee to formulate a boiler code, and that continues to be the main thrust of the ASME Boiler and Pressure Vessel Code to this day. If one wants to take a very simple approach to the change from 4 to 3.5, one need only ask whether a maximum increase of 14 percent in allowable stress values is justified based on the unprecedented advances in technology since 1944.
The advances in all facets of the technologies that affect the safe construction, testing, and operation of pressure containment components since 1944 have been revolutionary. We have come from what could be described as the dark ages, comparatively speaking, into modern times—a massive leap in the technology that impacts the safety of a pressure-containing component. We have learned how to produce clean steels, we can assure ourselves that a weldment is sound, and we can determine whether the structure is susceptible to an unanticipated brittle fracture. None of the procedures used to accomplish those goals were viable tools in 1944.
The conservativeness of Section I and Section VIII, Division 1, continues to be significant. The change from 4 to 3.5 in the material design factor will not compromise safety. The denominator for determining an allowable stress based on yield strength remains the same, 1.5. The probability of failure of a pressure-containing component due to an overstress condition is small. The change from 4 to 3.5 has had very little impact on that conservative material design factor.
The latitude between an allowable stress based on tensile strength and the ultimate strength of the construction material has been increased at most by about 5 percent at room temperature. In view of the magnitude of technology’s advances over the past 55 years, that small change in stress is inconsequential.