Stainless Stress Values
Selection of the Correct IID Listed Stainless Steel Stress Values
File: PVE-8296, Last Updated: Apr 8 2015, LB
This is an extract from one of our in house training topics, we thought our customers would find it useful.
Lots of IID Entries
The IID book provides many rows of stress properties for the 304 grade of stainless steel pipe. These 12 line items cover SA-312 304 stainless steel in welded, seamless, low, high and regular carbon and high and low stress. Quite a lot – here are the 3 variables covering the 12 lines.
- Seamless or welded
- Low or high strength
- The amount of carbon in the grade: L, regular or H.
This is further confused by Dual Certified stainless (304/304L). Other variations like N, LN also exist with different chemistry not discussed here.
For a simple 304 grade stainless pipe, the different combinations here provide for 12 different IID table 1A lines with different allowed stress/temperature curves. You only want one.
The table notes are required to choose the correct line. For Section VIII-1 use, if we do not care how the material properties are calculated, the following notes can be ignored:
G21 and W13 apply to section I only, W12 is for Section III use
T4, T6, T7 and T8 explain how the high temperature properties were calculated
These are the useful notes:
G3 These stress values include a joint efficiency factor of 0.85.
G5 Due to the relatively low yield strength of these materials, these higher stress values were established at temperatures where the short-time tensile properties govern to permit the use of these alloys where slightly greater deformation Is acceptable. The stress values in this range exceed 66% but do not exceed 90% of the yield strength at temperature. Use of these stresses may result in dimensional changes due to permanent strain. These stress values are not recommended for the flanges of gasketed joints or other applications where slight amounts of distortion can cause leakage or malfunction. For Section Ill applications, Table Y -2 lists multiplying factors that, when applied to the yield strength values shown in Table Y -1, will give allowable stress values that will result in lower levels of permanent strain.
G12 At temperatures above 1000°F, these stress values apply only when the carbon is 0.04% or higher on heat analysis.
G24 A factor of 0.85 has been applied in arriving at the maximum allowable stress values in tension for this material. Divide tabulated values by 0.85 for maximum allowable longitudinal tensile stress.
W14 These S values do not include a weld factor. For Section VIII, Division 1 and Section XII applications using welds made without filler metal, the tabulated tensile stress values shall be multiplied by 0.85. For welds made with filler metal, consult UW-12 for Section VIII, Division 1, or TW-130.4 for Section XII, as applicable.
Note G3 or G24 is applied to all welded pipes. The longitudinal efficiency for an ERW (Electric Resistance Welded or fillerless welded) pipe is set at 0.85. Note W14 indicates that a weld efficiency of 0.85 has not been included but should be included if the product is ERW.
It is obvious that the table would be simpler if the ERW and seamless product forms were separated. As a suggestion, try using the Smls. & wld. pipe grade only for seamless product, and Wld. pipe for welded grade. The additional 0.85 efficiency will not need to be applied resulting in less confusing calculation sets.
Even if an ERW efficiency factor of 0.85 has been applied, additional reduction in efficiency might be required according to the rules of UW-12(d). For example, pipe caps welded on the end of a vessel made out of ERW pipe with no radiography will require an efficiency of 0.85 to be applied to the pipe long seam, this is in addition to the 0.85 already taken off for the ERW efficiency. This is easier to do if the welded material line has been chosen from table IID. Here is an interpretation.
Standard Designation: BPV Section VIII Div 1
Subject Description: Section VIII, Division 1, UW-12(c)
Date Issued: 02/18/1988
Record Number: BC88-043
Historical Interpretation numbers : VIII-1-86-218
Question(s) and Reply(ies): Question: Is it the intent of the new stress multiplier rules that, for a vessel consisting of an ERW pipe shell with seamless ellipsoidal or torispherical dished heads and no radiography of the Category B seams, the stress values from Table UCS-23 for ERW pipe be multiplied by E = 0.85 for calculations involving circumferential stress in the shell?
Many customer drawings do not specify if the product form is seamless or welded. If this cannot be clarified on the drawing then the lower strength welded must be assumed.
High Strength or Low Strength
Half of the listing have note G5, indicating that the strength level of the material is set above the customary 66% yield limit. The use of these values is not recommended for flanges, but not prohibited. Our experience indicates that ASME VIII-1 Appendix 2 and Y flange designs are highly conservative. It is our policy to use high strength materials for these applications except when registering in the province of Alberta which has an undocumented requirement to use the low strength values, or when the customer prohibits it.
The Chemistry of the Grade (L, Regular or H)
Usually it is the job of the customer to specify the correct grade of stainless to use, but sometimes we have to ask for changes based on the operating temperature. The regular grade (TP304) has a maximum carbon content of 0.08%. TP304L has a maximum content of 0.035% and TP304H ranges form 0.04-0.010%
304L is not listed for applications above 1200°F. Note G12 prohibits the use regular grade 304 above 1000°F unless the carbon content is above 0.04%. (There is no explanation for the difference between 1200°F for the 304L grade and 1000°F for 304 without the extra carbon.) 304H always has 0.04% or greater carbon so no note is required for high temperature use.
Dual certified stainless is produced by the mill to meet the requirements of both 304 and 304L. From the above graph, the carbon content must be below 0.035%, and from the top table, the higher 75,000 tensile and 30,000 yield must be met. This is not a challenge for modern mills.
Sometimes customers use a confusing 304/304L designation on the bill of materials. Does the customer mean materials that meet both specifications (dual certified) which would allow higher strength levels to be used in the calculations, or is the customer giving themselves a choice between using the stronger or the weaker material, in which case the weaker would have to be calculated. Usually the customer means the first interpretation, but where this cannot be correctly interpreted, the weaker material must be calculated.
With these 3 variables understood, the most appropriate IID listed line can be selected. Other product forms like plate are simpler because the welded or seamless variable does not exist, but the method is the same.