Section 302.3.2 (d) of ASME B31.3 provides the basis of design stress or allowable stress for piping materials. As per this section, the basic allowable stress values at temperature for materials other than bolting materials, cast iron, and malleable iron shall not exceed the lowest of the following for temperatures below the creep range:

(1) the lower of 1/3rd of specified minimum tensile strength at room temperature ST and 1/3rd of tensile strength at temperature.

(2) except as provided in (3) below, the lower of 2/3rd of specified minimum yield strength at room temperature SY and 2/3rd of yield strength at temperature.

(3) for austenitic stainless steels and nickel alloys having similar stress–strain behavior, the lower of 2/3rd of SY and 90% of yield strength at temperature.

For temperatures above the creep range, the allowable stress will be the lowest of the following:

(4) 100% of the average stress for a creep rate of 0.01% per 1000 hr.

(5) 67% of the average stress for rupture at the end of 100,000 hr.

(6) 80% of the minimum stress for rupture at the end of 100,000 hr.

## Allowable Stress for Carbon Steel Pipe

The article takes you through a few examples to explain the above requirements. Let us take the example of ASTM A106 GrB seamless pipe. As per Table A-1, the specified minimum tensile strength is ST = 60ksi and specified minimum yield strength is SY = 35ksi.

Applying rule 1) and 2) above, the lower of

(1/3) of ST = (1/3)*60ksi = 20ksi

and

(2/3) of SY = (2/3)*35ksi = 23.3ksi

The lower of the two values is 20ksi. Hence the value of allowable stress is 20ksi from minimum temperature to 400°F. With respect to specified minimum tensile strength ST, this translates to a safety factor of 3 for ASTM A106 GrB pipe. ## Allowable Stress for Stainless Steel Pipe

Next, let us take the example of ASTM A312 TP316 pipe. As per Table A-1, the specified minimum tensile strength is ST = 75ksi and specified minimum yield strength is SY = 30ksi.

Applying rule 3) above, the lower of

(2/3) of SY = (2/3)*30ksi = 20ksi

and

(90%) of SY = (0.9)*30ksi = 27ksi

The lower of the two values is 20ksi. Hence the value of allowable stress is 20ksi from minimum temperature to 300°F. With respect to specified minimum yield strength SY, this translates to a safety factor of 1.5 for ASTM A312 TP316 pipe.

## Allowable Stress for Nickel Alloy Pipe

Finally, let us take the example of ASTM B444 UNS N06625 pipe. As per Table A-1, the specified minimum tensile strength is ST = 120ksi and specified minimum yield strength is SY = 60ksi.

Applying rule 3) above, the lower of

(2/3) of SY = (2/3)*60ksi = 40ksi

and

(90%) of SY = (0.9)*60ksi = 54ksi

The lower of the two values is 40ksi. Hence the value of allowable stress is 40ksi from minimum temperature to 300°F. With respect to specified minimum yield strength SY, this translates to a safety factor of 1.5 for ASTM B444 UNS N06625 pipe.

## Allowable Stress vs Temperature

Figure below shows a plot of Allowable stress in ksi vs Temperature in degrees Farenheit for ASTM A106 GrB, ASTM A312 TP304, ASTM A312 TP316 and ASTM B444 UNS N06625 pipe. ### Carbon Steel Pipe

For ASTM A106 GrB pipe, the allowable stress is (1/3) of tensile strength up to temperature of 400°F as discussed in above section. As the temperature goes above 400°F and up to 700°F, the allowable stress will be lower of (2/3) of yield strength and (1/3) of tensile strength at the applicable temperature. At temperatures above 400°F the allowable stress will be governed by (2/3) of yield strength rather than tensile strength. At temperatures above 700°F, the slope of allowable stress versus temperature becomes steepers suggesting a sharper decline in allowable stress with temperature. At temperatures above 700°F, the allowable stress is based on creep properties of ASTM A106 GrB pipe material.

### Austenitic Stainless Steel Pipe

For austenitic stainless steel pipe ASTM A312 TP316 pipe, the allowable stress is (1/3) of tensile strength up to temperature of 300°F as discussed in above section. As the temperature exceeds 300°F and up to 1050°F, the allowable stress will be lower of (2/3) of yield strength at room temperature and 90% of yield strength at the applicable temperature. At temperatures between 300°F - 1050°F, the allowable stress will be governed by a percentage of yield strength rather than tensile strength. At temperatures above 1000°F for ASTM A312 TP304 and 1050°F for ASTM A312 TP316, the slope of allowable stress versus temperature becomes steeper suggesting a sharper decline in allowable stress with increasing temperature. At temperatures above the elastic limits, the allowable stress is based on creep properties of stainless steel material.

### Nickel Alloy UNS N06625 Pipe

For nickel alloy ASTM B444 UNS N06625 pipe, the allowable stress is (1/3) of tensile strength up to temperature of 200°F as discussed in above section. As the temperature exceeds 200°F and up to 950°F, the allowable stress will be lower of (2/3) of yield strength at room temperature and 90% of yield strength at the applicable temperature. At temperatures above 200°F the allowable stress will be governed by a percentage of yield strength rather than tensile strength. At temperatures above 950°F, the slope of allowable stress versus temperature becomes steepers suggesting a sharper decline in allowable stress with increasing temperature. At temperatures above 950°F (elastic limits), the allowable stress is based on creep properties of ASTM B444 UNS N06625 pipe material.