Everyone wants a low figure for a piece-part tube or component, but after relaying a few dimensions they may absentmindedly omit that the tube must be coated or formed in a particular manner and with exacting precision. As a result, they may see a price based on incomplete information, but a higher final cost when all the particulars are finally taken into consideration.

Can you say sticker shock? To avoid this, a good first quote should be based on all design requirements and cost-cutting steps such as ordering in volume and moving toward more relaxed tolerances.

Cost-effective ordering

Although tubing has three dimensions - outside (OD), inside (ID), and wall thickness - only two need be specified. If ID is the more important dimension, tubing can be produced by mill to an ID and wall, or to OD and ID dimensions. Tolerances in any tube are applicable only to two cross-sectional dimensions. Thus, if outside diameter and wall thickness are specified, the inside diameter may not necessarily conform to published tolerances.

In addition, the right equipment lets facilities cut and deburr any diameter tubing in quantities from one piece to 100,000 or more, and from lengths as short as 0.040-in., and longer. A length tolerance of ±0.005 in. is standard on diameters of less than 1 in. Closer tolerances are possible, but those add costs because the process has to be closely monitored and therefore slowed down. Generally speaking, tighter tolerances add cost.

Making the grade

The stainless steel grade depends on the application. One grade may be better than others for certain environments and uses. Type 304 is the most common grade and is found in such applications as tooling for medical devices; 316 is the second most common grade and is used in moist environments, including invasive surgical devices; and types 321 and 347 are used in demanding applications such as aerospace. Typically, the higher the grade number, the higher the cost because the higher grades contain more alloying materials.

Most stainless tubes are produced with a bright silvery finish, although some parts come with a “pickled” or dull pewter finish. Most parts from this facility have finishes of the “bright” variety. It is always a good idea to specify the expected finish.

Stainless steel tubing is ordered to specifications that make the product versatile for general usage and capable of ready certification for more stringent requirements. These specs ensure that chemistry and mechanical property tests have been made, such as tensile, yield, and elongation, as well as flaring, flanging, hardness, flattening, and hydrostatic or nondestructive tests.

Welded stainless steel tubing is also ordered to meet quality standards for a multitude of uses. Welded sizes up to and including 5-in. OD are certified to ASTM A-249/A-269 (average wall). This specification ensures that tensile, yield, and elongation tests have been performed, in addition to flaring, flattening, flanging, hardness, and hydrostatic or nondestructive electrical tests.

Welded and drawn tubing are available in a size range of 0.202 to 8-in. OD and a 0.015 to 1-in. wall thickness. Welded tubing is cold drawn exactly the same way as seamless. As it undergoes repeated cold drawing and annealing it takes on the appearance and qualities of seamless tubing. The weld can be detected only by etching or by microscopic examination. Cold work and annealing recrystalizes the weld area with ductility and mechanical properties equivalent to the parent metal.

Cold-worked annealed tubing is available in an OD from 5/8 to 4 in. and wall thickness of 0.035 to 0.120 in. This latest manufacturing method starts in the same manner as welded and drawn tubing. Cold-rolled strip is rolled into tube form and fusion welded without the addition of a filler metal. It is then selectively cold worked in the weld area and given a full solution anneal that recrystallizes the area. With this controlled processing, the strength, ductility, and corrosion resistance of the weld area is equal or superior to the parent metal. This similarity between parent and weld zone structures gives cold worked, annealed tubing uniformity and makes it suitable for condenser tube and other heat exchangers.

Cold-worked annealed tubing meets the requirements of ASTM A-249 and tolerances of ASTM A-450. Required minimum mechanical properties - 75,000 psi tensile strength, 30,000 psi yield strength, and 35% elongation - are met in both the base metal and the weld area. The allowances in the table are nominal, but they are not necessarily adequate for all tubular products and machining methods. For example, when magnetic particle inspection or aircraft quality requirements are involved, greater allowances than those shown in the table should be used.

Of all the specifications, tensile strength has the least impact on pricing.

Clean up or machining allowances for round tubing

For machined parts size, OD, (in.)	Machined allowance on diameter (in.)
	Outside Diameter	Inside Diameter
Less than 3/32	0.008	0.008
3/32 to 3/16	0.012	0.012
2/32 to 1/2	0.015	0.015

Temper for 300 Series Stainless Steel

Condition	Ultimate tensile strength (ksi)	Yield strength (ksi)	Elongation % in 2 in. min	Rockwell hardness
Annealed	100	35 to 55	40	B90 max
¼ hard	100 to 110	55 to 75	35	B91 to C26
½ hard	110 to 140	75 to 125	15	C27 to C33
¾ hard	140 to 150	125 to 135	10	C34 to C38
Full hard	150 to 200	135 to 160	6	C39 to C43

Standard cutting tolerances

Diameters of (in.)	0.040	1
But less than	1	6
Tolerance is	±0.005	±0.015