Machine Design

Let The Chips Fly

Copper-rich alternatives to conventional AISI austenitic stainless steels sport similar mechanical and corrosion properties but are much easier to machine.

Robert S. Drab
Applications Manager
Ugine Stainless & Alloys Inc.
Doylestown, Pa.

Regulator-adjustment screw made from 304Cu stainless steel is used in the soda industry to regulate soda pressure.

An EGR valve shaft used in the automotive industry made from 304Cu.

A complex demonstration part that shows machining and formability properties of the copper-enhanced stainless-steel alloys.

Comparing the cold-work curve for T304L and its 304Cu (3.0 to 4.0% copper) counterpart shows that the work-hardening curve is retarded for the 304Cu as cold reduction rises. Work-hardening forces lattice displacements in the metal's microstructure, improving its hardness. But these displacements also induce residual stresses in the alloy that can be detrimental to machining and other forming operations.

Studies at Ugitech have shown that mechanical properties for a 303 alloy with 1.5% copper at equivalent cold-work reductions are slightly inferior to ordinary 303. The alloy will have some slightly lower properties in the annealed condition. But adjustments of cold work during processing can boost tensile and yield strengths.

Current density measurements of 303 and 304 stainless steels in an H2SO4 solution at different copper levels shows copper content improves the stainless steel's resistance to generalized corrosion.

This graph compares the effect of magnetic permeability with cold work for 303UX. Very little change in magnetic permeability is seen for work hardening rates up to 30%.

This graph compares the effect of magnetic permeability with cold work between 304Cu, 304L, and 316L. The increase in magnetic permeability of 304Cu is much less than 304L and 316L in strained conditions.

It's no secret that U.S. OEMs are increasingly under the gun to cut manufacturing costs to compete globally. One way to accomplish this is by sending labor-intensive machining jobs offshore. But there might be an advantage to instead look at material selection rather than labor alone as an alternative means of dropping part costs.

Most mechanical engineers in the U.S. are familiar with AISI-grade alloys. But designers currently using AISI T303 and T304 austenitic stainless alloys for fittings, bolts, screws, shafts, and bearings may want to take a hard look at the alloys their Asian and European counterparts are specifying for similar applications.

All carbon and stainless steels have hard inclusions in their matrix formed during the manufacturing process. These inclusions affect tooling during machining. Stainless steels in particular are notoriously poor for machining. They also tend to work-harden quickly, which makes them more difficult to form. Their chemistry and generally higher strength gives them a "gummy" nature that demands more machine-tool power and limits how fast they can be cleanly cut.

AISI T303 and T304 are the two commonly used stainless-steel alloys. They have similar chemistries with the basic difference being higher sulfur levels in T303. The added sulfur content makes T303 much easier to machine at conventional speeds and feeds than T304.

But increased sulfur levels come at a price. T303 is less resistant to corrosion. It is also more difficult to weld and sensitive to cracking in parts with thin cross sections. The AISI austenitic stainless steels also exhibit lower thermal conductivities and therefore don't efficiently dissipate the heat produced at the tool/chip interface. The machinability of T303 degrades when cutting temperatures approach 1,500°F, even with the added sulfur. In contrast, Asian and European designers for years employed two alloys that are variations of the workhorse AISI T303 and T304 grades. The alloys, 303Cu (303UX for the American version) and 304Cu, perform as well or better than their AISI counterparts in terms of mechanical properties and corrosion resistance. They also sport the added advantage of being easier to machine.

These alloys are made using a proprietary process from Francebased Ugitech S.A. (formally Ugine-Savoie Imphy) called Ugima that creates the steels without the hard inclusions responsible for the poor machining qualities of the AISI versions. With Ugima, the melt is engineered during the manufacturing process forming oxide inclusions with lower melting points. The inclusions are soft and malleable at high temperatures and serve a double role at higher and thus hotter machining speeds. They act as solid lubricants for the cutting tools, reducing tool wear. They also facilitate chip breakage by stretching along the shear zone. These properties let processors boost cutting speeds well above those of equivalent AISI grades. Additionally they greatly improve tool life, which in many applications is of greater benefit than faster cutting speeds.

Stainless alloys are primarily chrome, chrome-nickel, and chrome-nickelmoly-bdenumiron-based alloys with copper as a residual element (i.e., occurring naturally and not manipulated or specifically added during the alloy's melting process). AISI T303 and T304 do not have a copper specification in their alloy chemistry. But there are several current and older AMS/ASTM specs that do let alloy producers manipulate copper additions up to a maximum of 1.0%. It has been demonstrated that adjusting copper content toward the 1.0% limit retards the work-hardening rate and makes the alloy more ductile. And raising the copper content beyond the AMS/ASTM 1.0% limit reduces the work-hardening rate even further.

The copper levels in both the 303UX and 304Cu alloys are well above the AMS/ASTM 1% maximum at around 1.4 to 1.8% and 3 to 3.5%, respectively. Therefore, designers can't directly substitute them for T303 or T304, but can spec them as alternative alloys.

The addition of higher copper to the stainless steels brings many benefits. They include not only better ductility, reduced rate of work hardening, and ease of machining, but also better corrosion resistance and less magnetic permeability.

Better ductility enhances these alloys in a couple of ways. First, it reduces the necessary machining forces, leading to faster machining speeds and reduced tool wear. Second, better ductility helps with bendability. So alloys won't crack as easily during staking, swagging, and cold-forming operations. More bendability in the 303UX can also eliminate annealing operations typically needed after 303 alloys are machined then cold formed.

Further, the 303UX alloys tend to crack less in highly strained sections or when machined to small diameters and thin crosssection. Better ductility, along with the reduced sulfur levels, lets this occur.

This change in ductility is one component that leads to better machinability. Customer experience plus machining studies performed at Ugitech have shown productivity can rise as much as 60% over that possible with standard 303 grades. In addition, another study showed a 10 to 30% increase in productivity for 304Cu compared to 304 and 304L alloys.

Corrosion resistance is also bolstered by the addition of copper. This is especially true in oxidizing acidic media. During initial stages of corrosion, chemical reduction of the copper creates a metallic film on the metal surface. This film protects the metal from corrosive media and stabilizes the potential in the favorable zone corresponding to the chromium passivity.

The copper film reduces the corrosive potential of the reduction of positive hydrogen ions in the acidic media. In sulfuric acid, for example, boosting the copper content to 3% in a 303 alloy brings a nearly tenfold drop in current density.

Copper additions give 304Cu nearly the corrosion resistance of 316L in many aspects. There is a reduced rate of pitting and crevice growth, and good resistance to intergranular corrosion. Additional studies in other corrosive media such as salt spray have shown that though the additional copper does not reduce the initial formation of crevice or pitting corrosion, it does slow crevice and pit growth.

Magnetic permeability is another physical property that improves with higher copper level. It is well known that cold reductionof 303 and 304 causes slight increasesin magnetism, thanks in part to the transformation of austenite to martensite during cold working. Copper additions help reduce the magnetic permeability, yielding little to no growth in magnetic permeability in 303UX for up to 30% cold reductions. Additionally, 304Cu tends to have lower magnetic permeability than 304L in strained or cold-worked conditions at reductions up to 40%. This makes these alloys candidates for electrical, computer, and sensor applications where the low permeability will generate minimal magnetic interference.

Weldability is the final consideration for these high-copper stainless alloys. There is no known negative effect of copper in the weld zone or the weldability of 304Cu alloys. Weld properties are comparable to those of AISI counterparts with less copper. But as with T303 welding is not recommended for 303UX. If welding 303UX is unavoidable the best practice is to use ER312 welding wire as the filler metal to help minimize thermal cracking problems. Heat treatment must not take place after welding with ER312 wire.

In contrast, 304Cu can be welded with the same techniques as T304 including Stick, TIG, MIG, and lasers. And, no pre or postweld heat treatment is needed.

AISI/EN alloy chemical analysis






C (maximum)





Mn (maximum)





Si (maximum)





P (maximum)





S (minimum)






17 to 19

17 to 19

18 to 20

17 to 18


8 to 10

8 to 10

8 to 10.5

8.5 to 10



1.4 to 1.8


3 to 3.5

Ugine Stainless & Alloys Inc., (800) 523-3321,
Ugitech S.A., 33 (0)1 41 25 55 44,

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