Improving Machinability in 303 and 304 Stainless Steel ...

 

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Additional copper can improve the machinability of 303 and 304 stainless steel in a variety of ways.

 

U.S. firms that use T303 and T304 austenitic stainless steel alloys for applications such as fittings, bolts, screws, shafts and bearings will be interested in using improved versions of these alloys that are currently being used in Europe and Asia. These alloys from Ugine Stainless & Alloys, Inc. (Doylestown, Pennsylvania)&#;303UX (the U.S. designation) and 304Cu&#;are variants of 303 and 304 respectively with the addition of copper. The European EN spec is 1. for 303UX and 1. for 304Cu.

 

Figure 1 - Cold work curve for 304 and 304Cu.

The AISI/EN chemistries of these alloys are compared to standard 303 and 304 alloys in Table 1. Note that there is no copper specification in the AISI T303 and T304 chemistries, which is an indication that any copper present is a residual element&#;one occurring naturally and not manipulated or added during the melting process.

The Benefits of Adding Copper to 303 and 304 Stainless Steel

Copper, when added to stainless steel, provides numerous benefits to the metal. It has been demonstrated that adjusting this element toward its AISI limits retards the work hardening rate and increases the ductility of the alloy. Increasing the copper content beyond the AISI limit decreases the work hardening rate even further. The cold work curve plotted in Figure 1 (304 alloy versus a 304Cu alloy with 3 to 4 percent copper addition), shows the extent to which the work hardening curve is retarded for the 304Cu as cold reduction increases. The elevated copper content also provides enhanced corrosion resistance, reduced permeability and better machinability.

Several current and older AMS/ASTM specs allow copper levels to a 1 percent maximum for typical 303 and 304 alloys, allowing mills to manipulate this grade up to the maximum. The 303UX and 304Cu stainless steels have copper levels well beyond the 1 percent maximum; therefore, they cannot be used as direct substitutes for T303 and T304. They can, however, be considered as an alternative that offers all of the performance of 303 and 304 in addition to increased machinability, enhanced corrosion resistance, reduced magnetic permeability and better ductility or bendability.

Having better ductility enhances these alloys in a couple of ways. First, the increased ductility permits using reduced machining forces, leading to faster machining speeds and reduced tool wear. Second, the alloys&#; increased ductility help with bendability. That means there is less of a tendency for the alloys to crack during staking, swaging and cold-forming operations. Increased bendability in the 303UX can eliminate an annealing operation typically required when 303 is subjected to cold forming after machining.

The increased copper level of the 303UX also reduces the tendency for cracking to occur in small diameters, thin cross sections and highly strained sections. The increased ductility, along with reduced sulfur levels, allows this to occur. However, note that 303UX&#;s increased copper levels reduce&#;but do not eliminate&#;cracking tendencies.

Manufacturing Productivity Gains

This change in ductility is one factor that leads to increased machinability. Machining studies at Ugitech and customer experiences have shown that productivity increases of 30 percent or more can be seen using 303UX compared to 303XL alloy (and increases as much as 60 percent compared to the standard 303 alloy). Similarly, 10 to 30 percent increases for 304Cu compared to 304/304L alloy were also achieved.

The copper addition can enhance corrosion resistance, which is especially true in oxidizing acidic media. During the initial stages of corrosion, a chemical reduction of the copper leads to the formation of a metallic film on the metal&#;s surface. This metallic film directly protects the material from the corrosive media and stabilizes the potential in the favorable zone corresponding to the chromium passivity. Finally, it has been shown that this copper film reduces the corrosive potential of the reduction of positive hydrogen ions in the acidic media.

Additional studies in other corrosive media have shown that the increase in copper does not reduce the initial formation of crevice or pitting corrosion. However, it slows the growth of the pits and crevices.

Magnetic permeability is another physical property that is affected by copper level increases. It is well known that cold reduction of 303 and 304 alloys causes slight increases in magnetism, partly because of the transformation of austenite to martensite during the process. Copper additions help reduce the magnetic permeability, yielding little to no change in magnetic permeability for reductions of up to 30 percent.

One last consideration is weldability. Where welding is concerned, the high copper 303UX and 304Cu alloys are directly comparable to their conventional 303 and 304 counterparts. There is no known negative affect of copper in the weld zone or in the weldability of these alloys. They can be welded with the same techniques as T303 and T304 to yield the same results. Bear in mind, however, that all stainless steels with high sulfur content are difficult to weld, and alternative joining methods should be considered.

Grade

C Mn Si P S Cr Ni Cu

303

0.15

2.00

1.00

0.20

0.15

17.0

8.0

 

Max.

Max.

Max.

Max.

Min.

19.0

10.0

 

303UX

0.15

2.00

1.00

0.20

0.15

17.0

8.0

1.4

Max.

Max.

Max.

Max.

Min.

19.0

10.0

1.8

304L

0.03

2.00

1.00

0.045

0.03

18.0

8.0

 

Max.

Max.

Max.

Max.

Min.

20.0

10.5

 

304Cu

0.03

2.00

1.00

0.045

0.03

17.0

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8.5

3

Max.

Max.

Max.

Max.

Min.

18.0

10.0

3.5

This table compares the AISI/EN chemistries of the copper-rich 303UX and 304Cu stainless steel alloys with conventional T303 and T304 stainless steel alloys.

Remember that small additions of copper can enhance material performance, while maintaining mechanical properties of the AISI grades. Today, companies are concerned about cutting costs wherever possible. However, when it comes to specifying stainless steel alloys, most tend to select from the familiar set of traditional alloys instead of considering all of the possibilities. American manufacturing designers rely too heavily on American AISI, AMS and ASTM grades. They fail to consider newer offerings that can provide the same, if not better, performance characteristics and offer increased opportunities for reducing manufacturing costs.

Europe and Asia have enjoyed these alloys for many years and have been able to reduce costs without compromising part performance. The United States should start taking a serious look at these alloys to help reduce part costs and remain a viable player in today&#;s market.

&#; Schmolz + Bickenbach USA

How we choose to cook can be a personal experience based on a foundation of personal taste, memory, and more. What we cook food in, however, seems to change with the weather.

There are somany cookware options on the market that promise to be the best choice for cooking. You see all these buzzwords about the different materials and why they&#;re the right option. It can make it tough to keep up and know the differences.

Today is all about copper and stainless steel cookware and what makes each both good and bad.

At the end of this read, you should have a lot more background knowledge on these two materials that will help you make the best choices for your kitchen.

How Cookware Has Changed Over the Years

Cooking food in pots and pans dates back to 400,000 years ago. Back then, early humans used turtle shells and hollowed-out tree bark to cook food.

In fact, archaeologists hypothesize that food ended up cooking on a fire virtually by accident. In the beginning, before civilization took off, fires were used by humans to ward off potential predators.

We&#;ve discovered animal bones in many unearthed ancient fireplaces, meaning that meat ended up on the fire at some point. Anything that burned would have been used as fuel for the fire, so the genesis of cooking on a fire may have been a happy accident.

However it happened, we now cook meat over the fire relatively infrequently, choosing alternative heat sources like the range and the oven, and saving the open flames for special events like BBQs and outdoor events. Either way, the flavor combinations that chefs have come up with throughout the years make our mouths water.

Cooking Utensils Today

So now we find ourselves here &#; cooking our meats to a perfect medium-rare (warm red center) and arguing over which type of material is the best option for cookware.

While the options may seem limitless, today is only about two metals: copper and stainless steel.

We&#;ve got the pros and cons of each, and the information here should put you on the right path for your culinary endeavors.

What Are the Pros of Copper Pans?

Copper is one of, if not the oldest, metals used for cooking purposes. It was mainly used as a decorative element in homes, featuring in art and other decor.

Thankfully, someone finally added it to a fire and put some food in it.

Copper Heats and Cools Quickly

The first pro to copper cookware we want to discuss is its ability to heat up. It heats up quickly and evenly, making for a quality cooking experience.

It also loses heat rapidly, which may seem like a con but is necessary when you&#;re cooking delicate items, like fish or a sauce like bearnaise.

When you add the final ingredients and pull the pan from the heat, copper will immediately cool down, which helps prevent these items from overcooking or breaking due to continual cooking after removing the heat source.

However, this can also be a con, depending on what you&#;re cooking. For example, many chefs turn their noses at copper cookware due to its inability to retain heat for long. Searing steaks and other meats can be an issue due to how fast copper cools down.

Copper Has AntiMicrobial Qualities

Copper is also well-known for its natural antimicrobial qualities. The National Library of Medicine conducted a study in third-world countries due to the recommendation of using copper pots to store drinking water. The results favored copper.

The study saw drinking water stored at room temperature for 16 hours. At the end of the study, scientists found no evidence of bacteria growing in the water stored in copper pots.

Copper is naturally resistant to microbes, so the same effect will benefit you at home. This gives a lot of chefs piece of mind when cooking food due to the diminished risk of bacteria surviving or thriving on their cookware.

Copper Looks Amazing

There&#;s no doubt about it. Copper pans on display in the kitchen make the space feel glamorous and cool. This style of cookware is a showstopper that never fails to impress.

Many cooking shows utilize copper pans for decoration due to the luxurious look they have on camera. Even if you only have one or two pans in your kitchen, they&#;re likely to draw attention when guests walk into your home.

What Are the Cons of Copper Pans?

While the benefits of copper seem great, there are still downsides to copper that you have to mull over.

They&#;re not total dealbreakers, but you should give them some thought before making any serious purchases.

Copper Is Expensive

Copper might look amazing in your kitchen, but it will cost you. It is one of the more expensive options for your kitchen. That might be why it&#;s so impressive if you have it.

A few key things factor into the high cost of this material.

For one, you have to factor in the sheer cost of raw copper. It costs more than steel or aluminum, so copper cookware companies consider this when pricing their pieces.

Copper pans are also generally hand-made, driving up the labor cost for each pan, which drives up the selling price.

Lastly, copper pans are heavier than other cookware materials. This can be both good and bad for chefs and home cooks. The pan's weight will factor into the selling and shipping costs.

A big selling point is that copper pans are usually sold separately rather than as a set. The pro to this con is that you can buy one or two pans as a splurge and use them once in a blue moon.

If you need a reference, consider copper like a fancy sports car. It&#;s incredible, flashy, and draws attention, but it probably isn&#;t your everyday pan.

Copper Is Reactive

Copper reacts with highly acidic foods, like lemon juice or anything tomato-based.

This means that the pan will alter your food when you&#;re cooking items with those acidic ingredients for long periods of time.

Cooking with copper may impact the color and taste of your food. This means that food will darken when it is supposed to be light. You may also notice a subtle but distinct metallic aftertaste with every bite.

The pH of these foods is too high for the molecular bonds of copper, and your food and family will suffer the consequences.

Copper Is High Maintenance

Copper cookware is high maintenance.

It cannot be put into the dishwasher and must be hand washed. This hand washing process must be done with care &#; you must wash it delicately, by hand, and then polish it every time you use it.

This can be a pro for some people. Maybe the benefit of cooking with such a delicate and gorgeous piece of cookware comes with the labor of love that goes into caring for it afterward.

For most chefs, however, this process is too much for everyday use. Daily use isn&#;t recommended for copper, even if you are careful before and after use.

Even if you can afford an entire set of copper pots and pans, you need to weigh the care and maintenance of the cookware against the cost and determine if it&#;s worth it in the end.

What Are the Pros of Stainless Steel?

Stainless steel has a lot of benefits, and we want to talk about all of them.

Consider a copper pan a novelty or specialty item to keep in your kitchen. Stainless steel, however, can be more of a workhorse.

Stainless Steel Is Easy To Care For

It is pretty simple to take care of stainless steel pots and pans. They&#;re dishwasher safe and can be used daily with ease. This makes them an excellent option for those just starting in the kitchen and who cook all the time.

Novice cooks, even seasoned ones, make mistakes in the kitchen. It happens to all of us from time to time. But with a stainless steel pan, you don&#;t have to worry that the burnt dinner you served your family will be sticking around.

You can use sturdy brushes on these pans to get rid of food residues without worrying about distrupting the quality or integrity of the pan.

Stainless Steel Models Can Have a 3-Ply Design

There are plenty of options on the market for stainless steel cookware. However, those like Hexclad pans, which feature a 3-ply design, are in a category all their own.

The design of a 3-ply pan is a layer of stainless steel, one of aluminum, and another layer of stainless steel.

This works exceptionally as the aluminum core will conduct heat evenly, making hot and cold spots on the pan a non-issue. The stainless steel exterior ensures that you have a strong pan, resistant to scratching and chipping.

The last big benefit to a bottom layer of stainless steel is its ability to react to various heat sources. You can even use these pots and pans on an induction stove, which requires a magnetic pan that can react with the cooking surface to begin the heating process.

Not all stainless steel is created equal, and this design reigns supreme.

Check out our technology page if you would like more information about the design and innovation process that sets our pots and pans apart.

Stainless Steel Heats Evenly

It may seem tedious, but preheating a stainless steel pan makes for a better cooking experience.

Why?

A stainless steel pan looks smooth, but tiny pores are on the surface. When you preheat the pan, the pores of stainless steel react to the heat, shrink, and close up any gaps in the surface that you can&#;t see. This creates a smooth surface that&#;s great even heating.

Turn the stove on medium heat and allow the pan to warm up for about two minutes. Then add cooking oil, let it settle for a moment, and add food.

This will give you an even cooking surface that won&#;t burn the food or cause it to stick to the pan. It may sound like an extra step, but you will be happy with the results in the end.

Stainless Steel Is Durable

Stainless steel is a durable option. This means it&#;s perfect for students away at school, new chefs just starting, and those that cook day in and day out.

Unlike copper cookware, you can use stainless steel for every meal, every day. And as we&#;ve said, you can use just about the strongest brush possible to clean it without issue.

Another great feature of the durability of stainless steel is its nonreactive quality. This means you can make those acidic tomato and lemon sauces without the risk of discoloration and metallic flavors in your food.

Stainless Steel Is Cost-Effective

Compared to copper, stainless steel is the more affordable option.

The quality will vary with the price, so there may be something to the price difference from one company to another. But as a whole, stainless steel can be purchased in a full set of cookware pots and pans for a more reasonable price compared to other materials, especially copper.

What Are the Cons of Stainless Steel?

We know: With so many pros, what could steer you away from stainless steel? There are a few potential cons you&#;ll want to consider.

The two biggest cons of stainless steel are also the easiest to avoid with a bit of information and care.

Stainless Steel Has a Risk of Leaching

Even though stainless steel is non-reactive, there are instances when the metal can leach certain flavors into your food.

Leaching is the term used to describe foods altered due to the metallic surface of your pots and pans. This alteration happens with prolonged use at one time, so be mindful of how long you let your sauces and foods simmer in your pans.

As expected, leaching becomes less of a concern as long as you buy high-quality products.

Stainless Steel Comes in Different Grades

You can diminish the risk of leaching when using stainless steel, however, by only buying the right kind of stainless steel.

Stainless steel pots and pans come in different grades: 200, 300, and 400. These grades are associated with the amount of nickel contained within the material. In addition to the design of stainless steel pans, not all stainless steel is created equal.

200 grade stainless steel contains less nickel than 300 grade and is more durable than 400 grade.

Grade 300 for stainless steel is most common for surgical equipment as it is the least corrosive and can withstand saline (saltwater) solutions.

Grade 400 is the cheapest option. It has more carbonand is less resistant to corrosion. It is also the least expensive, so be mindful of this when you&#;re selecting cookware.

You can save money with stainless steel versus copper, but don&#;t be cheap with stainless steel.

The Bottom Line

Both of these cookware materials offer benefits as well as some setbacks. Cooking at home with your family is an opportunity to make great food and memories you take into the future, so what you cook with needs to fit with your lifestyle.

The information here, and our personal recommendations, should come in handy, helping you make the most informed decision possible.

 

Sources:

Storing Drinking-Water in Copper Pots Kills Contaminating Diarrhoeagenic Bacteria | PMC

What&#;s a &#;Nonreactive Saucepan&#; and Why Does It Matter? | Edmonton Journal

Cookware and Bakeware | New World Encyclopedia

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