ASTM tubular material specifications are grouped into two tier levels; the General Specification and the Product Specification. The General Specification describes detailed requirements such as testing, tolerances, and references common to more than one of the Product Specifications. The Product Specifications focus is covered in the Scope. The Product Specification identifies what requirements are needed and any special needs different than that required in the General Specification. If in conflict, the Product Specification requirements dominate.

In the listing below, the Product Specifications are arranged with the most demanding requirements at the top. This may not always be true as additional customer requirements and supplementary more restrictive requirements may also be included on purchase order requirements.

A 270 This specification is focussed toward seamless & welded tubing with for Sanitary applications. In most cases, a special surface finish is required. If a borescope inspection is required, A 1015 was developed to address this.

A 803/ A 803M This specification addresses welded ferritic stainless steel for feedwater heater and u-bent tubing for heat exchangers. Code adopted

A 688/A 688M Similar to A 803 except for welded austenitic stainless steel. Code adopted.

A 249/A 249M Welded and cold worked welded austenitic stainless steel for heat exchanger and boiler applications. This is the straight tube specification patterned for Code applications.

A 213/A 213M Similar applications as A249 except seamless only and covers low alloy, ferritic stainless, and austenitic stainless grades. Code adopted

A 268/A 268M Welded heat exchanger and general purpose ferritic and martensitic stainless steel. Code adopted

A 789/A 789M Welded and Seamless heat exchanger and general purpose duplex stainless steel. Code adopted.

A 632 Welded and seamless small diameter austenitic stainless steel for general purpose

A 269 General purpose welded and seamless austenitic stainless steel not requiring cold work. As it has no mechanical property requirement, this material is not intended for Code application.

A 554 Welded stainless steel for mechanical applications.

A 358/A 358M Filler metal welded austenitic stainless steel pipe. Code adopted with special qualifications.

A 409/A 409M Welded large diameter (NPS 14 to 30) austenitic stainless steel pipe. It may be welded with or without filler metal. Code adopted with special qualifications.

A 814/A 814M Welded and cold-worked non-filler metal austenitic stainless welded pipe. This is a higher quality than A312, intended for flanging and bending quality.

A 813/A 813M Non-filler metal added single or double welded austenitic pipe with tighter OD tolerances.

A 312/A 312M Seamless and welded austenitic stainless steel pipe. In general, this specific does not allow filler metal, except for restricted repair. This specification is very commonly used and is accepted by the Code.

A 790/A 790M Seamless and welded duplex stainless steel pipe. Code adopted.

A 778 Welded austenitic stainless steel pipe without anneal. This specification is not accepted by the Code.

Q. How do I find Tolerances and Testing Requirements in ASTM Specifications?

ASTM tubular material specifications are grouped into two tier levels; the General Specification and the Product Specification

The Product Specifications include a list of the specific requirements for the product identified in the scope. When a requirement for something like a tolerance appears to be missing from the Product Specification, the requirements from the General Requirements are to be used. When a conflict occurs, the requirements of the Product Specification override the General Specification.

A number of stainless steel alloys meet this criteria.  They can be an austenitic alloy, such as AL6XN, a ferritic alloy such as SEA-CURE, or a duplex.  By comparing the chromium, molybdenum and nitrogen content using the formula %Cr+3.3 times % Mo+16 times % N, an approximate temperature can be estimated using the figure below.  The results of the formula is commonly referred to as the pitting resistance equivalent number, PREN.

Q. What Alloys are Resistant to MIC Corrosion?

Microbiological Influenced Corrosion (MIC) is normally the result of a deposit or slime containing certain microbes that produce acid as a waste product.  The deposit or slime traps the acid against the metal surface resulting in a very aggressive environment that would normally not be envisioned by reviewing bulk water chemistry.  

Alloy considered resistant to MIC are those that have a minimum crevice corrosion temperature of 29 degrees C when measured by the ASTM G48 crevice corrosion tests. These alloys included the 6% molybdenum containing austenitic stainless such as AL6XN, super duplex stainless steels, SEA-CURE, and titanium grade 2. SEA-CURE is currently the most cost effective alloy of the group.

Passivation of stainless steels is the formation of a chromium enriched surface layer that helps to protect stainless steel.Although stainless will self passivate in air at ambient temperature, the diffusion kinetics are very slow that this layer is only marginally protective. Contaminants, such as iron on the surface or manganese sulfides from the casting operation, can create local galvanic corrosion cells that are unable to self passivate. These are areas where pitting can initiate.

Chemical passivation is the use of a chemical or electrochemical (such as electropolishing) method to dissolve contaminates on the surface that prevent a defect free protective layer. The most commonly used solutions are oxidizing acids, such as nitric or phosphoric. Other acids, such as citric acid, have also been used successfully. Details of these solutions can be found in ASTM A380 and ASTM A967. Additionally, tests to determine the success of these treatments are included in ASTM A967.

Passivation of stainless steels is the formation of a chromium enriched surface layer that helps to protect stainless steel.Although stainless will self passivate in air at ambient temperature, the diffusion kinetics are very slow that this layer is only marginally protective.Contaminants, such as iron on the surface or manganese sulfides from the casting operation, can create local galvanic corrosion cells that are unable to self passivate. These are areas where pitting can initiate.

The iron can be from a variety of sources, such as contact with steel tools or bands, or insufficient removal of a heat tint or heat treat scale. The scale or tint develops at high temperature and scavenges the available local chromium leaving a chromium depleted (or iron rich) layer below. This prevents proper repassivation.

Mechanical removal of the scale, tint, or chromium-depleted layer does not restore equivalent corrosion resistance.  Iron rich areas can be re-embedded into the surface developing new local corrosion cells. Only a chemical or electrochemical (such as electropolishing) technique can remove the embedded contaminates. In some cases, the technique can concentrate the chromium on the surface providing even greater corrosion resistance.

Three components are needed for Stress Corrosion Cracking (SCC) to occur: a minimum stress level, a sufficient amount of corrodant, and a minimum temperature. All three of these are interrelated and are also a function of the chemistry of the alloy.

Eliminating the stress is normally not an option as the stress is a combination of all sources including residual, fabrication caused, pressure induced, thermally induced, and other sources such as hanging loads. Minimizing all of these is almost impossible.

Chloride levels below 5 ppm have been known to cause failure of TP 304. Higher levels reduce the time to failure.

TP 304 has a minimum threshold temperature around room temperature. Failure for this grade is unlikely below 20 degrees Celsius. As the temperature increases, the potential increases and the time to failure decreases.  Failure has been known to occur in a few hours when the alloy is exposed to high levels at high temperatures. 

Initial work by Copson and Chang determined that the potential for failure and susceptibility is directly related to the nickel content of the alloy. The most sensitive alloys contain 8% nickel. This also happens to be the nominal nickel content of TP 304 and its derivatives. TP 316, containing nominally 10% nickel, is only slightly more resistant. For good resistance for SCC, choose a grade with very low nickel or a
nickel content of 25% or greater. The low nickel grades include TP430 and TP439 when the solution is only mildly corrosive and SEA-CURE if chloride levels are above 500 ppm.

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1. The most common source for stainless steels is chlorides, although in sufficient quantities, caustics and polythionic acid can also be a corrodant.  Depending upon stress levels, temperatures, and operation modes that can cause concentration, less than 1 ppm chloride in the bulk water has been found to initiate cracking.

2.  The stress could be from a variety of sources, including applied stress, thermally induced stress, and residual stress.  The combination of all of the sources is what drives the cracking.

3.  Depending upon the alloy content, cracking can occur at just above room temperature. Alloys that are more resistant have higher initiation temperatures.

The element with the greatest effect is nickel. Copson and Chang determined that alloys with 8% nickel (type 304 and its derivatives) are the most sensitive. TP 316, containing just over 10% nickel, is only slightly more resistant. Sensitization from improper heat treatment, carbon contamination, or subsequent welding can increase sensitivity. To get significant resistance, alloys with less than 5% nickel or more than 25% nickel need to be chosen. In the power industry, it is common to select TP 439 for applications less than 550 F maximum temperature, and a 6% molybdenum alloy such as AL6XN, when the temperature may exceed 550 F.

Many people believe that all ferritic alloys or duplexes containing a ferrite matrix are immune to chloride SCC. This is not true. Although these alloys are more resistant than TP 304, they can crack in certain conditions. The only stainlesses that are considered to have excellent resistant to cracking have less than 0.5% nickel and are not sensitized.

Although the term is commonly used, no universally accepted definition exists for stainless steels. The most common opinion is that it is a heat treatment to dissolve detrimental chromium carbides and cooling quickly enough to prevent them from reforming. Chromium is the element that provides the protective layer on stainless steel that makes it Stainless. When chromium is in the form of a chromium carbide particle, it is not available to form the protective layer. Most often, this happens near grain boundaries and intergranular corrosion and attack can result.

Other definitions are also used. Complete homogenization of the alloy can be another expectation. As diffusion rates of elements other than carbon can be much slower, complete homogenization may not be possible with the common heat treatments used for solution anneal. For some of the more corrosion resistant austenitic alloys containing molybdenum, a substantial coldworking operation followed by a longer furnace anneal may be required to provide maximum corrosion resistance.

Plymouth Tube Co. maintains an extensive library of technical articles in this area.

Pickling is the chemical removal of surface contaminants from corrosion resistant materials such as stainless steels or nickel alloys. Most commonly, this cleaning is used to remove the oxide scale that develops during heat treatment. However, it is also sometimes used after welding to remove heat tint or as a general cleaning when the stainless may have been exposed to iron. Surface contaminants, such as iron or iron oxide on the surface or manganese sulfides from the casting operation, can create local galvanic corrosion cells that are unable to self passivate. These are areas where local attack can initiate and propagate.

The most commonly used solutions for stainless steels and nickel alloys are oxidizing acids, such as nitric/HF or phosphoric. Other acids, such as citric acid, have also been used successfully. Details of these solutions can be found in ASTM A380 and ASTM A967. Additionally, tests to determine the success of these treatments are included in ASTM A967.

A chemical pickling operation can result in higher chromium to iron ratios on the surface than in the base metal. This can promote improved corrosion resistance.

Q. Can a welded tube have advantages over a seamless product?