May 20,2004Page 1 of 27
R 26
6.11Materials
6.11.1General
6.11.1.1Materials of construction shall be selected for the operating and site environmental conditions specified (see 6.11.1.7).
Discussion: Key materials concerns are mechanical properties and corrosion resistance. The purchaser may know of requirements or stream contaminants not listed on the data sheets. There may also be differences of opinion between the purchaser and supplier on the suitability of materials for the specified process and site environments.
6.11.1.2The material specification of all major components shall be clearly stated in the vendor's proposal. Materials shall be identified by reference to applicable international standards, including the material grade (refer to informative Annex XXX) Where international standards are not available, internationally recognized national standards may be used.When no such designation is available, the vendor's material specification, giving physical properties- chemical composition, and test requirements- shall be included in the proposal. [9.2.3, Item k]
Discussion: National Standards such as ANSI, DIN, BS are examples of internationally recognized national standards. Internationally recognized “other standards” such as API, HIS NEMA, AGMA, etc. may also be used.
6.11.1.3If specified, copper or copper alloys shall not be used for parts of machines or auxiliaries in contact with process fluids. Nickel-copper alloy (UNS N04400), bearing babbitt, and precipitation-hardened stainless steels are excluded from this requirement.
Note: Certain corrosive fluids in contact with copper alloys have been known to form explosive compounds.
Discussion: There is potential of an explosive mixture occurring under certain conditions. For example, ethylene oxide in the presence of copper can form acetylene. Nickel-copper alloys (such as Monel and its equivalents), bearing babbitts and precipitation-hardening stainless steels also contain certain amounts of copper. However, the presence of nickel in these materials acts as a barrier to the process of formation of explosive mixtures.
6.11.1.4The vendor shall specify the optional tests and inspection procedures that may be necessary to ensure that materials are satisfactory for the service (see 6.11.1.2). Such tests and inspections shall be listed in the proposal. [9.2.3, Item j]
Note The purchaser can specify additional optional tests and inspections- especially for materials used for critical components or in critical services.
[The use of the word “may” is not appropriate for use in a NOTE since it implies “permission” to perform a requirement, and requirements are not allowed in a NOTE. The use of the word “can” is used to indicate a possibility and is therefore not a requirement and is appropriately used in a NOTE. [ISO Directives Part 2 Annex G paragraph G.3].
Note to TF Chairmen: Check to be sure there is space on the data sheets to specify this option.
Discussion: Material specifications often contain appropriate optional mechanical or chemical analysis tests and optional inspections as supplementary requirements. These requirements are considered suitable for use with each material specification aid should not surprise the supplier. For critical castings, for instance radiography of certain areas may be justified. Carbon equivalent (carbon or carbon with other elements) maximums are sometimes specified to improve weldability and to reduce hardness at welds.
6.11.1.5External parts that are subject to rotary or sliding motions (such as control linkage joints and adjustment mechanisms) shall be of corrosion-resistant materials suitable for the site environment.
Discussion: Corrosion - resistant materials are necessary to prevent binding or seizure. Consider exposure to intermittent contaminants from wash down water, nearby process or cooling water leakage sources, and process gas leaks, for example.
6.11.1.6Minor parts such as nuts, springs, washers, gaskets, and keys shall have corrosion resistance at least equal to that of specified parts in the same environment.
Discussion: Minor parts often perform critical functions and must be corrosion resistant to maintain their integrity. Fasteners may be higher strength than other components and therefore are more susceptible to stress corrosion cracking.
Non-ferrous materials often have lower melting points than steel, with reduced fire resistance.
6.11.1.7The purchaser shall specify any corrosive agents (including trace quantities) present in the motive and process fluids and in the site environment, including constituents that may cause stress corrosion cracking. (SPTF are there other corrosion mechanism which should be identified)
Note Typical agents of concern are hydrogen sulfide, amines. bromides, iodides, chlorides, cyanide. fluoride, mercury, naphthenic acid and polythionic acid.
6.11.1.8If austenitic stainless steel parts exposed to conditions that may promote intergranular corrosion are to be fabricated, hard faced, overlaid or repaired by welding, they shall be made of low-carbon or stabilized grades.
Note: Overlays or hard surfaces that contain more than 0.10% carbon can sensitize both low-carbon and stabilized grades of austenitic stainless steel unless a buffer layer that is not sensitive to intergranular corrosion is applied.
6.11.1.9Where mating parts such as studs and nuts of austenitic stainless steel or materials with similar galling tendencies are used, they shall be lubricated with an antiseizure compound suitable for the process temperatures and compatible with the material(s) and specified process fluid(s). (ISO – David Sales)
Note The required torque values to achieve the necessary bolt preload will vary considerably depending if antiseizure compounds are used on the threads. . [6.2.8, 6.2.9.4]
Discussion: Some antiseizure compounds have been found to play a role in promoting stress corrosion cracking under certain conditions. For example, the combination of molydisulfide thread lubricants and humid air can cause SCC problems in A193 B7 materials. The molydisulfide decomposes at elevated temperatures to form corrosive hydrogen sulfide. Also, sulfur-based, copper-based and lead-based lubricants can contribute to cracking of materials such as l 7-4PH and cold-worked and annealed 304 SS.
6.11.1.10When the purchaser has specified the presence of hydrogen sulfide in any fluid, materials exposed to that fluid shall be selected in accordance with the requirements of NACE Standard MRO 175. Ferrous materials not covered by NACE MR0 175 shall not have a yield strength exceeding 620 N/mm2 (90,000 psi) nor a hardness exceeding Rockwell C 22. Components that are fabricated by welding shall be postweld heat treated, if required, so that both the welds and the heat-affected zones meet the yield strength and hardness requirements.
It is the responsibility of the purchaser to determine the amount of wet H2S that may be present, considering normal operation, startup, shutdown, idle standby, upsets, or unusual operating conditions such as catalyst regeneration.
In many applications, small amounts of wet H2S are sufficient to require materials resistant to sulfide stress corrosion cracking. When there are trace quantities of wet H2S known to be present or if there is any uncertainty about the amount of wet H2S that may be present, the purchaser shall note on the data sheets that materials resistant to sulfide stress corrosion cracking are required. [ Note made part of the paragraph since it specifies requirements ]
Discussion: NACE MR0175 (2003 is the latest edition) lists ferrous and non-ferrous materials that are resistant to sulfide stress corrosion cracking. The owner can also use MR0175 to specify materials resistant to sulfide cracking for environments not specifically defined in that standard NACE is the only widely recognized standard that exists today.
Sulfide stress corrosion cracking only occurs when moisture (water) is present with the H2S. In many petrochemical applications the combination of moisture and H2S may occur during normal operation or during transient conditions (such as startups and shutdowns). The cost of complying with this requirement is often relatively low compared to the benefits realized.
Post weld heat treatment accomplishes two things: l) tempering back hardened (martensitic) transformation products produced during welding and 2) stress relief of any induced tensile stresses during welding.
6.11.1.7 The purchaser shall specify any agents (including trace quantities) present in the motive and process fluids and in the site environment, including constituents that may cause stress corrosion .
Note to Task force chairmen: Modify these form of corrosion based on your standard.
Note1: Seven common forms of corrosion are :
1) General corrosion
2) Pitting corrosion
3) High temperature corrosion
4) Intergranular corrosion (IGC)
5) Environmental corrosion
6) Selective attack
7) Erosion corrosion
Note 2: Environment corrosion is the brittle fracture of a normally ductile material in which the corrosive effect of the environment is a causative factor.
Note 3: Environment cracking caused by environmental corrosion is a general term which includes the terms listed below.
1) Stress Corrosion Cracking (SCC)
2)Sulfide Stress Corrosion Cracking (SSC)
3) Chloride Stress Corrosion Cracking
4) Corrosion Fatigue (CF)
5)High Temperature Hydrogen Attack
6)Hydrogen Embrittlement (HE)
7)Liquid Metal Embrittlement (LME)
8)Hydrogen Blistering
9) Hydrogen Induced Cracking (HIC)
10) Stress Oriented Hydrogen Induced Cracking (SOHIC)
Note 4: Stress Corrosion Cracking (SCC) is the most dangerous of the various types of corrosion failure of metals. SCC occurs unexpectedly and is extremely localized. As a rule, SCC is accompanied by little change in the equipment wall thickness. During SCC, the metal or alloy is virtually unattacked over most of its surface, while fine cracks progress through it. There is no obvious correlation between the amount of corrosion and cracking due to stress corrosion cracking. SCC can cause through fracture in very short periods of time (in the most severe cases in a day or even several hours). SCC is minimized by mimimizing residual stresses, proper material selection and by limiting the hardness of the material.
Note 5: Typical agents of concern for environmental cracking are hydrogen sulfide, amines. Halides (bromides, iodides, chlorides, fluoride) chlorine, cyanide. fluoride, mercury, naphthenic acid, polythionic acid, hydrofluoric acid, mercury, carbon dioxide, ammonia, ammonia bisulfide, phenols, caustics (sodium, potassium and lithium hydroxide) , sea water, brine.
Note 6: The documents referenced in 6.11.1.8, 6.11.1.14, and 6.11.1.16 through 6.11.1.19 cover corrosion due to sulfide, chloride, caustic and alkaline stress corrosion cracking. Purchaser and vendor are advised to consider mitigation processes to cover the other form of corrosion outlined in Notes 1& 3 which may caused by the process fluid.
6.11.1.8 If the purchaser has specified the presence of hydrogen sulfide in any fluid, materials exposed to that fluid shall be selected and processed in accordance with the requirements of NACE Standard MRO 175103.
Discussion:
Cause: Atomic Hydrogen entering the steels microstructure.
Prevention: Limit strength and hardness per NACE MR0175 & MR0103 and ISO 15156.
Background: The Standard Paragraph Task Force is in the process of evaluating referencing MRO 175 and 103. MRO 715 “Metals for Sulfide Stress Cracking
and Stress Corrosion Cracking Resistance in Sour Oilfield Environments” has been referenced for many years in the SOME Standards, even though its title referenced “oil field”.
ASME P-Numbers and Weld Procedures
To reduce the number of welding and brazing procedure qualifications required, base metals have been assigned P-Numbers by Section IX of the ASME BPVC (Boiler Pressure Vessel Code) .These assignments are based essentially on comparable base metal characteristics, such as composition, weldability, brazeability, and mechanical properties, where this can logically be done.
Within P number categories for steel and steel alloys (i.e. P-Numbers 1-11) the base metals are further broken down into subset categories called Group Numbers. P-Number 1 has 4 sub Group numbers. P-1 and 4 groups are listed below.
P number 1: Carbon or carbon-manganese steels
– Group 1: Minimum tensile strength of less than 70 ksi
– Group 2: Minimum tensile strength of 70–80 ksi
– Group 3: Minimum tensile strength of 80–90 ksi
– Group 4: Minimum tensile strength of greater than 90 ksi
From ASME Code Section IX Table QW-422
There are 98 materials listed in P 1- Group 1
There are 48 materials listed in P 1- Group 2
There are 14 materials listed in P 1- Group 3
There are 3 materials listed in P 1- Group 4
NO IMPACT TEST OF WELD REQUIRED: The ASME Boiler Pressure Vessel Code Section IX indicates a procedure qualification (WPS) developed for one of the materials in a P number category can be used for all the materials in that P number and all Groups associated with that P number. Thus for P 1 category, a weld procedure qualification (WPS) developed for a material in P1 Group 1 can be applied to all of the materials in P1 Groups 1,2,3 and 4. i.e one weld procedure can be applied to 98+48+14+3= 163 materials.
IMPACT ESTING REQUIRED: A separate WPS has to be developed for each Group associated with a P No. For P1 therefore, you would need 4 separate weld procedures, one for each of the Groups. The WPS developed for Group 1 could be used for all materials P1 Group 1 but not for Groups 2, 3 or 4.
If impact testing is required, and you wanted to weld a material from P1Group 1 to a material in P1 Group 2, you would need to develop a separate weld procedure. This weld procedure could then be applied to welding any material from P-1 Group 1 to any material in P-1 Group 2. (Refer to QW 403.5 Section IX of the ASME Boiler Pressure Vessel Code)
The ASME Boiler pressure vessel code approach is based on strength and structural integrity. In addition to the structural integrity, however when invoking NACE we are also concerned with the hardness of the weld ( this includes the weld metal, HAZ and base material). Structural integrity AND resistance to Sulfide Stress corrosion Cracking is required. For the materials in P 1 groups 1, 2 and 3 NACE historically ( and SP as modified) requires the weld metal, HAZ and base metal to be less than HRC 22.
The final weld hardness, in part, depends on the Carbon Equivalent of the base metal, trace or micro-alloying elements in the base material , weld filler material and post weld heat treatment.
The affect of the CE and trace metals are not covered by the ASME Boiler pressure vessel code.
CARBON EQUIVALENT: Generally the higher the amount of carbon in the base material, the harder the weld. The weld hardness is also increased by alloying elements such as Mn, Ni, Cr, Mo and V. The formula used by NACE MR0103 states:
CE=%C + (%Mn/6) + [(%Ni + %Cu)/15] + [(%Cr + %Mo + % V)/5]
There can be considerable difference in carbon equivalent between the 98 P1 Group 1 materials. Therefore a qualification procedure which was developed for a P1 Group 1 material with a CE of 0.35 may produce RC readings below HRC 22, but another P1 Group 1 material with a CE of 0.45 may result in harnesses greater than HRC 22. If the P1 Group 1material with the CE of 0.35 was used to qualify the entire P1 Group 1 category it is possible to get hardness readings exceeding HRC 22 if the material has a CE greater than the CE 0.35 used in the qualifying procedure
TRACE MICRO-ALLOYING ELEMENTS: As the result of using scrap steel in the manufacturing of “New Steel” certain trace elements are introduced. Trace elements which affect the hardness of the Heat Affected Zone are Nb(Niobium), V (Vanadium) and Cb (Columbium). Note: Nb and Cb are the same element.
The following is from the article “Vanadium and Columbium Additions in Pressure Vessel Steels by P.Xu, B.R. Somers and A.W. Pense
“…The maximum harness in the HAZ increases with increasing additions of V and Cb…”
“… Post weld heat treatment must be used with caution in High Strength Low alloy steels with V and/or Cb because it enhances the HAZ hardness but causes detrimental effects to the HAZ toughness.
Please note that the amount of these trace elements are not covered in the ASTM material specifications.
NACE SP 0472 ( Referenced as part of NACE MRO103)
NACE SP0427 addresses the issue of CE and Trace Micro-alloying elements in paragraphs 2.3.5.6.2.& 2.3.5.6.3 reproduced below.
2.3.5.6.2 The WPS shall state that the maximum CE of the production base metal shall not exceed the CE of the procedure qualification specimen by more than 0.03%. The base metal chemistry of the procedurequalification specimen shall be reported in the PQR. All base metal chemistry requirements shall be applied to ladle analyses, unless otherwise specified by the user.
2.3.5.6.3 For product forms in which deliberately added microalloying elements (such as Nb [columbium {Cb}], V, titanium [Ti], and boron [B]) are used, the maximum content shall not exceed the corresponding value onthe procedure qualification specimen. Deliberate additions are generally considered to be values greater than 0.01 wt% for each of Nb (Cb), V, and Ti, and greater than 0.0005 wt% of B. All base metal chemistry requirements shall be applied to ladle analyses, unless otherwise specified by the user.
It is for the above reason that NACE MR0 103 has been referenced in the SP. The CE and trace element requirements are not on NACE MR0 175.
Discussion : ISO 15156-1/ MR0175 states “ This part of ISO 15156 is not necessarily applicable to equipment used in refining or downstream processes and equipment.”
6.11.1.9 The hardness referenced for ASME Sec. IX P-No. 1 group 1 and 2 carbon steels in NACE MRO 103 – 2007 and NACE SP0472-2008 shall not exceed Rockwell hardness HRC22 in the weld, heat affected zone and base material. In accordance with ASTM E 140, HRC22 is equivalent to Brinell hardness HBW 237 and Vickers hardness 248 HV.
NOTE: Refer to NACE MRO SP0472 Appendix A, and NACE 8X194 for additional information concerning hardness testing and hardness limits.
Discussion: NACE SP0472-2008 require the referenced hardness not to exceed HRC 15 (HBW 200) . “The lower limit was applied to compensate for both the nonhomogeneity of some weld deposits and the normal variations in production hardness test results that are obtained using a comparison hardness tester.”
NACE 0472 Appendix A
“A2.2.1 A number of SSC failures occurred in the late 1960s in hard weld deposits in P-No. 1 steel refinery equipment. The petroleum refining industry established a maximum hardness limit of 200 HBW for P-No. 1, Group 1 and 2 steels to ensure that weld deposits would be resistant to HSC. The 200 HBW
maximum hardness requirement is lower than the 22 HRC (237 HBW) maximum hardness requirement listed in NACE MR0175/ISO 15156 and previous editions of NACE Standard MR0175. The lower limitwas applied to compensate for both thenonhomogeneity of some weld deposits and the normal variations in production hardness test results that are obtained using a comparison hardness tester.”