Metallurgical aspects of tube production
Understanding the science, improving the manufacturing
Small-diameter tubing plays a crucial role in many markets, including aerospace, nuclear, medical, and industrial. From coronary stents to hydraulic aircraft controls, each application has unique requirements. To meet the requirements of customers in these industries, well-designed processing steps and adequate control are critical.
The metallurgical aspects of small-diameter tubing (5/8 inch and smaller) can be divided into three categories:
- Raw material characteristics
- Process design and control
- Finished tube inspection and testing
These aspects are linked to and dependent on each other, so looking at them as a group can help produce a favorable result: a product that meets or exceeds customers' demands.
Procurement of raw material is more than merely issuing a purchase order. At a miniumum, the material must meet industry specifications defined by a credible organization such as ASTM or the Society of Automotive Engineers (SAE), which publishes Aerospace Material Specifications.
In addition to this basic requirement, several other technical criteria may be necessary for successful and efficient tube production.
Chemistry. For example, the cobalt content in many stainless steels may be restricted to a maximum of 0.10 percent by weight to meet a standard set by the nuclear industry. Gas transmission pipelines for the semiconductor industry can have a minimum sulfur content of 0.05 percent. Some aerospace applications require a restricted ferrite number in 321 stainless.
Purchasing and stocking material with restricted chemistry allows tube producers to procure a single material for many applications, which can help to reduce inventory levels and decrease customer lead-times. The material might cost more, but the advantages can be greater than the costs in many cases.
Inclusion Rating. Nonmetallic inclusions are identified as Types A, B, C, and D, which represent sulfide, alumina, silicate, and globular oxide. These are undesirable remnants of the melting and refining processes. A lower inclusion level is associated with a higher quality level. However, a lower inclusion level also is associated with higher cost.
For most applications, inclusion levels of Level 2 Thin and Level 1 to 1 1/2 Heavy in all four types are considered acceptable. Nonmetallic inclusions are more critical for light-wall tubing production because they cause splits, cracks, and surface ruptures.
Titanium carbonitrides, although not rated on a routine basis, may cause serious problems in very light-wall tubing.
Mechanical Properties. Softer material (material with lower yield strength and hardness and higher elongation) generally is more suitable for cold working than harder material. Tube and strip producers can make materials softer by using higher annealing temperatures or longer soak times. However, these can cause larger grain size.
Grain Size. For most materials, a grain size of ASTM #5 or finer is adequate. In nickel and cobalt alloys, some limits should be set for duplex structures (for example, a grain size difference of no more than 2 within the same microstructure).
For materials used in manufacturing light-wall, small-diameter tube involving multiple draw/anneal cycles, the starting grain size is not very critical because the processing significantly alters the grain size. In such a case, it is possible to start with a grain size #3 and end up with #7. For material that needs only two or three draw cycles, the starting grain size must be much closer to the ideal grain size for the finished product.
Microstructure. Austenitic stainless and nickel-cobalt alloys should be free of grain-boundary precipitation. Generally, a precipitation rating C1 is desirable, and C1-C2 is considered acceptable. Anything worse should be subjected to metallurgical review.
Carburization (top) and intergranular attack (bottom) usually are caused by inadequate cleaning before annealing in upstream processes. These conditions can be remedied by removing some surface material by processes such as grinding or grit blasting. If left alone, these conditions provide sites for trapping more contaminants and get worse with each cold-work and anneal cycle.
All materials must be free of carburization and intergranular attack (see Figure 1).
Process Design and Control
In addition to the starting stock size and quality, a well-designed process takes into consideration cold-work preparations, modes of cold work, cleaning, annealing, postanneal cleaning, and subsequent operations as needed (see Figure 2).
Preparation for Cold Work. Tube must meet straightness, dimensional tolerance, surface quality, and temper (annealed) requirements for successful cold reduction. In addition, if a surface preparation step such as grit blasting is used, the remaining grit on the ID must be completely removed. Any coatings used in conjunction with drawing lubricants must be evaluated for low-melting-point materials such as tin, cadmium, boron, lead, bismuth, and zinc.
Modes of Cold Work. For breakdown steps in the process cycle, large reductions in both OD and wall thickness are common. Compressive reductions, such as those performed by tube reducers or pilger mills, are effective for such reductions. Drawing on a draw bench is another option. However, maximum reductions in draw bench operations are only about 60 percent of the maximum reductions achievable on a pilger mill. Also, bench draw preparation steps, such as pointing and applying coatings, add cost and time to a project, and the bench drawing process chews up the tube ends and can result in significant amounts of scrap.
The best overall economy is achieved by finding the optimum combination of these processes.
Cleaning After Cold Work. ASTM A380 is a comprehensive standard that recommends general practices for cleaning. However, each manufacturer must develop its own cleaning process—one that removes all drawing compounds and lubricants effectively. Oil content and solid residue content limits can be set by experimentation and must be strictly followed.
Typical manufacturing steps used for producing small-diameter tubing include repeating steps 1–5 until the tube is the finish draw size, then a single iteration of steps 6–9. Step 9 is not used if manufacturing tubing for coils.
Regardless of the process, equipment, or techniques used, tube must be thoroughly cleaned to prevent problems associated with contamination, carburization, and intergranular attack.
Process Annealing. The goal of process annealing is to render the cold-worked material as soft as practical so the next cold-work step can be performed without any difficulty. It is common practice to use higher annealing temperatures for austenitic materials and longer soak times for martensitic and, to some extent, ferritic materials.
Postanneal Cleaning. Tubing annealed in bright-finish hydrogen furnaces or vacuum furnaces may not need any postanneal cleaning, but open-air anneals with water quenching usually result in oxide and scale formation. These must be removed by pickling or mechanical conditioning.
Nitric and hydrofluoric acid solutions are effective in cleaning stainless steel and nickel and cobalt alloys. Adequate washing afterward is required to remove the pickling residue to prevent intergranular attack. Alloys that resist pickling require mechanical conditioning. In such cases, the cleaning process must remove all grit before further processing.
Finish Draw. Depending on the mechanical property requirements, surface finish required, and dimensional tolerances, the choices are mandrel draw (rod draw and plug draw) or sink. For coils, the two methods are floating plug draw and sink.
Final Cleaning. The criteria for final cleaning are similar to those for in-process cleaning. However, final cleaning has one significant difference in that it affects tube placed in long-term storage.
Although chlorine-based solvents are used extensively, the use of aqueous solutions is becoming more widespread.
Residual chlorides are unacceptable because they can cause stress corrosion cracking. Additional cleaning steps with deionized water with high resistivity usually are sufficient to remove residual chlorides. In some critical applications, such as manufacturing nuclear reactor components, a pickling treatment, a passivation treatment, or both are performed and then followed by cleaning with deionized water.
Final Annealing. Most stainless and nickel alloys are annealed according to industry standards. Within recommended ranges, suitable temperatures, times, and cooling rates are used to achieve desired properties.
Straightening. Six-roll rotary straighteners are commonly used, but 10-roll straighteners are gaining ground because of their capability to apply incremental pressure and single or double flex. Products with higher OD-to-wall ratios tend to straighten better with 10-roll straighteners. Two-plane bar straighteners tend to be more suitable for heavy-wall tubing.
Each straightener setup affects the mechanical properties of the tube in different ways. Customers should be made aware of any changes in the setup so they can evaluate the effect of these on their downstream processes.
In the entire production process, three areas of metallurgical controls can make or break the product. While all aspects of process control are important, these three are the most critical.
Mechanical Property Control. The percentage of cold work is the main determinant of mechanical properties. Small tubing in austenitic stainless and nickel and cobalt alloys is produced in 14 hard, 12 hard, and 34 hard tempers, in addition to annealed temper. In recent years 18 hard temper also has gained popularity among fabricators. This temper usually retains a substantial amount of fabrication capability while adding some strength and machinability. This temper has been widely accepted for nuclear and medical applications because the corrosion resistance is fully retained despite the cold work.
Grain Size and Microstructure Control. Grain size can be controlled by adjusting the annealing time and temperature and percentage of cold work. Many medical products and convoluting grades of tubing call for grain size #7 or finer. While this is easily achieved for stabilized grades of stainless such as 321 or 347, it can pose formidable challenges in 316L or INCONEL® alloys. Several steps of the draw-and-anneal cycles need to be adjusted to achieve finer grain size.
For convoluting grade tubing, customers also specify a minimum number of grains across the wall to prevent the orange peel effect after convolution. Also, the grain size plays an important part in any tube that requires ultrasonic inspection. The microstructure must be free of precipitation in grain boundaries for austenitic stainless steels. The cooling rate during the annealing process is the critical factor to achieve this.
Control of Cleaning Operations. Preanneal and postanneal cleaning steps help prevent carburization, intergranular attack, and sites for entrapment of cleaning fluids and pickling residues. Also, residual chlorides can cause stress corrosion cracking eventually, whether the tube is in service or in storage.
Product Inspection and Testing
Inspection and testing to ensure conformance to order requirements are integral parts of the manufacturing process. In fact, some in-process inspection and testing steps are built into the process design stage itself. For example, when the finished tubing ID surface has to be completely free of microstructural contamination and intergranular attack, it is necessary to perform in-process checks on samples by metallography.
Surface removal techniques such as grit blasting and pickling can be used to correct these conditions. It may even be beneficial to make removing a small amount of material from the ID a standard procedure before the final draw. The evaluation then can be used as a monitoring tool.
For finished tubing, the inspection and testing can be broadly classified into nondestructive and destructive tests. Nondestructive tests include dimensional measurements, visual inspection, surface roughness check, hardness test, chloride check, fluoride check, hydrostatic test, ultrasonic test (UT), and eddy current test.
The hardness test usually is considered noncritical for austenitic grades of material. For martensitic grades, which often are used in hardened and tempered condition, hardness usually is achieved by pretesting to set time and temperature cycles for hardening and tempering.
For UT, the grain size of the finished tubing is critical. Usually grain size #6 or finer is necessary for UT to be successful. If the grain size is #7 or greater, noise from the tubing may interfere with defect interpretation. Most of the ASNT guidelines and customer specifications limit the acceptable noise level.
The chloride and fluoride checks ensure the effectiveness of cleaning and the subsequent removal of the cleaning solutions.
Destructive tests include tensile tests, manipulation tests, metallography (grain size and microstructure check), and a chemistry check.
Tensile Tests. Tensile tests are the most common of all product requirements. Annealed and straightened products usually do not pose a problem in meeting the specifications, but tubing with intermediate tempers (such as 1/8, 1/4, or 1/2 hard) need more attention. A tensile test in the last step of the drawing process and again during straightener setup usually are required, especially if the limits are tightened by the customers. This is especially critical in light-wall tubing for medical or aerospace applications.
Manipulation Tests. Manipulation tests—including flaring, flattening, flanging, reverse flattening, and reverse bending—ensure fabrication capability. Normal manufacturing practices are adequate for meeting standard manipulation tests. Challenges come when a customer requires, for instance, a flare of 70 percent or a flange of 75 percent on tougher grades. Such challenges can be met by:
- Using a higher annealing temperature during the final anneal.
- Reducing the amount of cold work to alter the range of mechanical properties (lower yield strength and higher elongation) in the final product.
- Using a stress-relieving treatment after the final straightening of the tube.
Metallography. Metallography is used to ensure the tube conforms to specifications related to grain size, precipitation rating, defect level, and general microstructure (including freedom from intergranular attack, carburization, and intergranular oxidation). Metallography also is a suitable tool for in-process evaluation and final diagnosis of rejects. Evaluating the rejects can lead to a better understanding of the causes of the flaws, enabling corrective actions for subsequent lots.
Chemistry Check. Nuclear, aerospace, and medical end users are sensitive about changes in chemistry (especially concerning carbon and sulfur and, in reactive metals, elements such as oxygen, hydrogen, and nitrogen). A finished tubing analysis is required in all such cases. Like metallography, a chemistry check can be used to determine the causes of flaws so that processes can be adjusted to eliminate them.
An Overview of the Processes
Reviewing the metallurgical aspects of every step in manufacturing small-diameter tubing and understanding how each step affects other steps are critical to a successful outcome. Because manufacturing small-diameter tubing requires a large number of individual yet inter- related processes, planning each step of each process carefully and monitoring the product at crucial stages can help a manufacturer improve its operations by learning from errors and preventing them in future lots. Careful planning and thorough monitoring can help to achieve successful manufacturing and improve efficiencies, improving the company's bottom line.
Chiranjib Mukherjee is manager of metallurgical services for Superior Tube Co., 3900 Germantown Pike, Collegeville, PA 19426-3112, 610-489-5371, fax 610-489-5333, firstname.lastname@example.org, www.superiortube.com.
INCONEL is a registered trademark of Special Metals Corp.
American Society for Nondestructive Testing (ASNT), P.O. Box 28518, 1711 Arlingate Lane, Columbus, OH 43228-0518, 614-274-6003, fax 614-274-6899, www.asnt.org.
ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, 610-832-9585, www.astm.org.
Society of Automotive Engineers International (SAE), 400 Commonwealth Drive, Warrendale, PA 15096-0001, 724-776-4841, www.sae.org.
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