ASME STP-PT-080-2016 pdf download.DEVELOPMENT OF AVERAGE ISOCHRONOUS STRESS-STRAIN CURVES AND EQUATIONS AND EXTERNAL PRESSURE CHARTS AND EQUATIONS FOR 9CR-1 MO-V STEEL.
1.1 Introduction The intent of this report is to describe the development of a creep model for use in producing isochronous curves for grade 91. The basis for the other component of strain, the plasticity or “hot tensile” curve, has been described elsewhere. Values of creep strain are needed over a wide range of conditions. At some temperatures, stresses, and times, the creep strain is dominated by the primary component; at other conditions tertiary creep is important. The model must in some cases be predictive of conditions for which there are no available data, specifically estimating creep strains at very low stresses, high temperatures, and long times. In describing the model, we try to maintain a distinction between terms such as condition, parameter, constant, and coefficient. The conditions are the inputs to the model: stress, temperature, and time. The parameters of the model are the values that are used to describe the shape of the creep curve at a specific set of conditions. For example, the stress exponent, n, is a parameter, and the time to rupture, t r , may also be considered a parameter. The model coefficients are used in describing the parameters as functions of stress and temperature. The term constant is only used for specific coefficients that take on a special role in a time-temperature parameterization. In this report, the term constant is exclusively used for the Larson- Miller constant. It is highly desirable to keep the number of parameters low to minimize the effort of determining the coefficients. Many have come to view the classic three stage description of creep as the result of a primary stage where hardening mechanisms result in diminishing creep rates and a tertiary creep stage where damage and aging mechanisms produce an increasing creep rate. The second stage, where creep rate appears to be constant, is simply the transition between the two stages. Primary-tertiary forms for creep models often involve four parameters, two each for the primary and tertiary stages.
Inspection of the creep curve shapes would seem to be a simple way of distinguishing between power law and logarithmic rate-type behavior. The published NIMS datasheets provide creep curves in different formats for a variety of heats designated as MGA, MGB, MGC, MgC. Over a particular range of either time or strain, curve fits from either form may provide very reasonable results. It can be observed for the NIMS curves that the power law form for the primary creep works well for most curves over several orders of magnitude in time (Figure 1.1(a) and (b)), but at very small strains it tends to break down as a limit to the creep rate is reached (Figure 1.1(c)). The logarithmic rate-type formulation generally holds for the tertiary portion of the curve, but some curves tend to bend down slightly (Figure 1.1(d)). In primary creep, it is difficult to find an appropriate region to perform a regression for logarithmic rate parameters.ASME STP-PT-080 pdf download.