phone-icon800.354.0806
765.456.6012
The Home of HASTELLOY® and
HAYNES® Alloys

20741
For Better Corrosion-Resistant Welded Tubing

A Distinction Between "Bead Worked" and "Cold Reduced" Tubular Products

Many fluid process designs require tubular products for use as piping and in various heat exchanger systems. When aggressive process streams are involved, corrosion-resistant alloys (e.g. nickel-base alloys) are selected. Since weldments are usually the most vulnerable to corrosive attack (Figure1), seamless tubing is often considered. However, cost considerations steer the designers to the selection of welded tubing that has been cold worked and annealed. The cold working plus annealing steps are performed to provide complete recrystallization and substantial homogenization of the seam weld and thus approach performance similar to that of a wrought seamless microstructure.

Most welded high-molybdenum nickel-base alloy tubing is procured to specification ASTMB 626 class III. The class III designation covers cold-worked and solution annealed prod-uct.* Within this class designation, however, the method and amount of cold working the seam weld prior to annealing is left up to the buyer. Different tube mills use different techniques to accomplish the cold working step. “Bead-working” or “bead rolling” (i.e., rolling only the seam weld to introduce some degree of cold-work) is a common method practiced by stainless steel mills to supply tubing to ASTM B626 specifications. In contrast, “cold drawing” or “cold pilgering” (i.e., cold reduction of the seam weld and the tube wall) is the preferred technique used by specialty mills for the production of nickel-base alloy tubing.

20742 The “bead worked” type and the “cold reduced” type tubing are both covered by the same classes (IIA, IIB and III) of ASTM B 626 specification. To some procuring personnel, the only difference between both types appears to be a few thousand dollars of cost saving. While such a saving might be applicable to stainless steel tubing, it has been proven devastating for plant operation when HASTELLOY® alloy tubing is required to handle aggressive chemicals. Many “bead worked” generic alloy C-276 tubes have failed prematurely at the seam welds in various corrosive services. Some were scrapped (Figure 2) and were replaced by “cold-drawn” HASTELLOY tubing, within the same ASTM specification. Thus, while the original purchasing decision may have been made on the basis of cost saving, such experience confirmed to the plant personnel that the saving was a delusion, for far greater cost was incurred from the unanticipated shut down and the replacement of the unit.
* See Appendix I for further descriptions of ASTM B 626 classes I, II and III.

20743Effects on Corrosion Resistance of HASTELLOY Alloys

Even though HASTELLOY tubular products are highly alloyed with chromium, molybdenum and tungsten to withstand aggressive chemicals, they need to be in an “optimum” metallurgical condition in order to provide the anticipated resistance to corrosion (comparison between two different metallurgical conditions is illustrated in Appendix II). When the HASTELLOY alloys are supplied in “non-optimum” conditions, such as the case of “partially” recrystallized and “nonfully” homogenized seam weld, corrosive attack is very likely to initiate and propagate along the weld (Figure 3). Failure analyses conducted on several “bead-worked” tubes and pipes confirm that preferential corrosion often initiates at the seam weld (whether ID or OD, Figure 4). 20744The corrosive attack propagates along “segregated”/”depleted” zones within the “partially” recrystallized structure of the “bead worked” seam weld. Perforation and leaks can ensue.

Metallurgical Considerations

For material ordered to ASTM specification B 626, the classes II A, II B and III are usually relied upon to procure welded tubing with an optimum recrystallized and homogenized seam weld microstructure for enhanced resistance to corrosion. In theory, “bead working” or “bead rolling” the seam weld should provide an adequate amount of cold work of the weld cast structure. Complete recrystallization and substantial homogenization of the seam weld into a wrought structure should take place in the solution annealing step. For many stainless steel welded tubing, “bead working” often yields the desired microstructure and corrosion resistance within the applicable ASTM specifications.20745
However, it is common knowledge that high-molybdenum nickel-base alloys are much more difficult to cold-work than stainless steels, and that minor irregularities are usually encountered in the seam weld. Such weld irregularities and deformation characteristics stand in the way of achieving a completely recrystallized microstructure, via “bead working” of welded HASTELLOY alloy tubing. In addition, the issue of interdendritic segregation of high-molybdenum nickel-base alloy welds is an important one and a carefully selected heat treatment cycle (time and temperature) is a must in order to achieve the desired homogenization of the seam weld. Even with near-perfect seam welds, the “bead working” might lead to slight undercut at the weld fusion line. These undercut regions could act as crevice corrosion sites in aggressive media. While many HASTELLOY alloys provide enough resistance to corrosion at such potential crevice sites, it is the partial recrystallization and the lack of full homogenization of the seam weld that represent the major concern for optimum corrosion resistance. In practice, perfect seam welds and good bead working are not always achieved consistently in high-molybdenum nickel-base alloys. In fact, “off symmetry” seam weld have been encountered in tubular products. 

20746

In such cases, only some zones of the weld get the proper amount of cold deformation via bead working while other zones may not get cold worked at all. As such, the annealing step would lead to partial recrystallization of the seam weld (Figures 5, 6 and 7). Additional schematics of cross-sections of bead-worked tubing are illustrated in Appendix III. 
20747

Impact on End Users

Consistent quality levels with optimum metallurgical conditions are sought by plant personnel when using welded tubular products. In theory, procurement to ASTM specifications should lead to assurance of consistent, well-defined metallurgical conditions. However, the ASTM specifications leave the option to the buyer to select between the cold working of the weld-metal only (bead working) or of cold working both the weld and the base metal (cold-reduction via drawing or pilgering).

As such, additional detailed specifications are required to insure optimum metallurgical conditions for best resistance to corrosion. To some “equipment specifiers”, inconsistent metallurgical conditions in seam welds might be avoided by relying on traditional appraisal techniques, such as inspection. However, inspection does not always prevent tubing with undesirable metallurgical characteristics from being delivered to end users. Such cases have occurred with devastating consequences to plant operations (the following passages).
20748

*Tubing with weld defects may not necessarily be rejected at an inspection test for quality control (Eddy current test, ultrasonic test). This is inherent to the inspection procedures, especially when the defect produces a signal that is smaller than the "acceptance percentage" of the model defect signal set at calibration.

Haynes International Tubular Products Arcadia Plant

Keeping in mind that quality is not to be inspected into a product, rather it is built into the product from its earliest stages of production, major end users prefer the consistent quality of “cold reduced” and annealed tubing. Many share the concern that seam welding might vary from batch to batch, that bead rolling will not consistently produce the appropriate cold worked condition for complete recrystallization, and that proper annealing step is required for homogenization of the seam weld. Haynes International, Inc., has recognized the problems associated with welded high-molybdenum nickel-base alloy tubing/ piping since the early 70’s. In response to the plea of many end users for improved quality, a tubing mill was built in Arcadia, LA, in 1976 with full dedication to quality HASTELLOY alloy welded tubes and pipes.

20749The Arcadia Specialty Mill relies on the “cold drawing” or the “cold pilgering” process to produce consistent quality HASTELLOY alloy tubing supplied to ASTM B 626 class III (cold-worked, annealed, and non-destructively tested). Unless the end user insists, Arcadia avoids quotations and shipments of welded tubing to ASTM B 626 classes I A and I B (sized and annealed). This is due to the fact that all standard tubing processes at Arcadia are set to achieve optimum metallurgical conditions, using cold-reduction of both the seam weld and the tube wall.     

APPENDIX I

ASTM B 626 Specification

Description of Scope, Manufacture and Nondestructive Testing Requirements

Paragraphs* referenced from ASTM B 626:

1.3 Five classes of tubes are covered as follows:

1.3.1 Class lA -Welded, sized, solution annealed, and nondestructively (NOT) tested in accordance with 10.5.1

1.3.2 Class IB- Welded, sized, and solution annealed. (No NOT required.)

1.3.3 Class IIA- Welded, cold worked, solution annealed, and nondestructively tested in accordance with 10.5.1.

1.3.4 Class liB- Welded, cold worked, and solution annealed. (No NOT required.)

1.3.5 Class Ill- Welded, cold worked, solution annealed, and nondestructively tested in accordance with 10.5.2.

4.2 Subsequent to welding and prior to final heat treatment, Class II and Class Ill tubes shall be cold worked either in both weld and base metal or in weld metal only. The method and amount of cold working may be specified by the purchaser. When cold drawn, the purchaser may specify the minimum amount of reduction in cross-sectional area or wall thickness, or both.

10.5.1 Class lA and Class IIA Tubes - Each finished tube shall be subjected to the hydrostatic test, the pneumatic test, or the eddy current test at the manufacturer's option.

10.5.2 Class Ill Tubes- Each finished tube shall be subjected to the pneumatic test and the eddy current test. Tubes larger than 1-1/2 in. (38.1 mm) in outside diameter may be subjected to the hydrostatic test in lieu of the pneumatic test at the manufacturer's option.

*Reprinted, with permission from the Annual Book of ASTM Standards, copyright American Society for Testing and Materials.

APPENDIX ll

207410

207411

APPENDIX Ill

Schematics illustrating examples of cross-section appearances of some "bead worked" high molybdenum nickel-base alloy tubular products.

207412

Print Page
Back to top