Machining of stainless steels

Stainless steel is a material that is difficult to machine. Machining entails a risk of cold tempering. In addition, machining errors can create attack surfaces for corrosion. The following article answers the frequently asked question "Can stainless steel rust?", provides an overview of problems during machining, various machining methods and selection criteria for the correct tool for machining stainless steel.

What is stainless steel?

The term stainless steel refers to alloyed or unalloyed steels with a special degree of purity. Colloquially speaking, stainless steel refers to all corrosion-resistant steels; however, in the technical sense, stainless steel can most certainly rust. DIN EN 10020 defines the terminology for steels, including stainless steels.

Stainless steels are popular materials due to their availability and properties. Stainless steel has the following properties:

• Durable
• Visual appealing
• Very variable

The versatility in particular is very appealing: Stainless steel has varying properties depending on its composition. For example, chromium ensures corrosion-resistance while nickel promotes acid-resistance. Due to the positive properties that can be adjusted by different alloys, various stainless steels are often used to manufacture standardized parts for mechanical engineering.

The following figure shows a comparison of the steel properties by stating the designation iaw. JIS and the associated material codes valid in Europe:

Comparison of the properties of different steels

• A = Corrosion resistance
• B = Polishability
• C = Machinability
• D = Weldability
• E = Magnetizability
• The further outward the parameter value is shown, the more pronounced it is.

The following steels are encoded by the material number: X5CrNi18-10 (1.4301 / SUS 304), C X X105CrMo17 (1.4125 / SUS 440), X8CrNiS18-9 (1.4305 / SUS 303) and 100Cr6 (1.3505 / SUJ 2).

Problems with handling and machining corrosion-resistant steel

It is incorrectly assumed that corrosion-resistant steel is 100% stainless. This is not true: a passive layer or oxide film that protects the steel against corrosion forms under the influence of oxygen on the surface of chromium-alloyed stainless steel.   This layer is very thin at 0.06 - 0.08 micrometers. However, it takes a certain time for this layer to form and to actively protect against corrosion. Typical problems during machining are that not enough time was allowed for the layer to form or the passive layer is destroyed by scratches, including during machining. However, contact with other less precious metals can also lead to rust (contact corrosion) of a steel that is actually classified as stainless. The less precious metal reacts upon contact, for example with water and oxygen. It corrodes. It extracts the oxygen required for oxidation from its immediate environment. The passive layer of the chrome-alloyed stainless steel consists of a chromium oxide, a chemical compound of chromium and oxygen. At the contact point, the chromium oxide layer is thus deprived of the oxygen and the protective layer is destroyed as a result. The now unprotected surface of the stainless steel can then also corrode.

Various precautions for handling and machining stainless steels

Due to the aforementioned problems, a lot of attention must be paid when handling and machining corrosion-resistant steel. For example, the tools used for stainless steel should be used exclusively for this purpose and should have no prior contact with other steels. This includes the storage of the tools.

However, things can still go wrong even after the corrosion-free steel has made it through the machining operation. If stainless steel comes into contact during transport, for example with iron on forks on forklifts or tools, the resulting stains can present a new corrosion risk. Special care must therefore be taken here as well to ensure that only suitable means of transport are used.

But now let’s look at the machining process itself: The following describes various machining methods, what needs to be observed, and what tools to use.

Cutting and parting stainless steel

Due to the above problems, a lot of attention must be paid when parting and cutting stainless steel. The cutting process can generate heat or create airborne rust. Machining methods, such as drilling and milling, often present a challenge when machining stainless steel. For example, the alloy element nickel has a negative effect on cutability and machinability of the steel. In addition to using special high-performance drills and milling tools, it is also important use machining parameters that match the material. As a rule, an abrasive aid is therefore added when cutting stainless steel to size. The following is a brief overview of common methods for cutting stainless steel:

• Water jet cutting: An extremely fine water jet is directed under high pressure (up to 6000 bar) onto the metal by means of a nozzle together with an abrasive material such as sand. The sand simultaneously polishes the surface, thus ensuring a low surface roughness. No heat is generated by this cutting method. This method is also suited for thick sheet metal. However, compared to other methods, it is relatively slow and the costs for the additional abrasive can be high depending on the sheet thickness.
• Plasma cutting: An electrically conductive gas (plasma) is likewise directed onto the metal by a nozzle, where an arc is created between the electrode and the workpiece, which strongly heats up and melts the surface. However, a relatively wide kerf joint is created in comparison to laser or water jet cutting. However, the advantages are that a wide variety of contours can be implemented, thick sheet metal can be cut and that the method is generally very fast.
• Laser cutting: A highly focused laser beam is directed onto the metal, which is then accurately melted/evaporated. The heat-affected region is very limited. Laser cutting can be used to manufacture complex contours; it parts stainless steel without wear and is very precise and fast. However, laser cutting is less suited for thick sheet metal.

Joining and connecting stainless steels

Corrosion-resistant steels can be joined and connected in different ways. The suitable methods depend greatly on the material and its properties. Possible joining methods are:

• Welding
• Soldering
• Gluing

In addition to the fact that not all stainless steels can be welded, welding in particular presents the greatest risk of corrosion. The resulting strong heat can lead to the formation of chromium carbides, internal tensile stresses or tempering (oxidation). If cracks develop, the probability of gap corrosion is high. The welding beads should only be laid down with suitable materials and welding methods and should always be passivated after the welding process. This is the only way to ensure corrosion resistance. Because the stainless steel surface is passivated, a flux agent is required during brazing. If stainless steel is to be glued, the surface must first be roughened. The smoother the surface, the weaker the bond of the adhesive material. Because if its high hardness, special tools are required for drilling stainless steel. MISUMI for example has the following tools for machining the various materials:

• Circular saw blades
• Hole cutters
• End mills

Surface treatment and surface machining: Grinding and chemically passivating stainless steel

The stainless steel surface can for example be brushed, sanded, polished or peened. This changes the surface roughness of stainless steel. The rougher the surface, the more susceptible stainless steel becomes to corrosion. Grinding is therefore a popular method for surface treatment of metal.

Common methods for grinding stainless steel include belt grinding and precision grinding. Belt grinding creates particularly smooth and high-quality finishes and larger material removals can be realized. However, various surface roughnesses can also be realized by using different grain sizes in the abrasive. High-quality grinding belts are crucial to prevent contamination with foreign rust. For this reason, MISUMI has a wide range of different abrasives for stainless steel processing.

Precision grinding as a further method aims to achieve maximum accuracy; the wet grinding process in particular is very precise. The resulting surfaces are particularly flat and parallel. For example, this can play an important role with regard to tolerances, see also the blog article Processing limits and accuracy standards for sheet metal parts as well as tolerance classes iaw. ISO 22081 and DIN ISO 2768: Optimized use of general tolerances. Find more details about this in the meviy portal.

Tool selection

For efficient and successful machining, much can already be taken into account when selecting the tools. Tools for machining stainless steel should themselves be hard, but not too hard. Tool that are too hard can become brittle quickly and break more easily as a result of machining. In addition, increased vibrations can occur during machining, which affects the surface finish. Determining the correct hardness of the tool is therefore essential for stainless steel processing. The hardness for stainless steel and the machining tools is generally based on the Rockwell (HRC) scale. Typical HRC values for stainless steel machining tools are in the range of 58 to 65 HRC.

Other hardness ranges apply, depending on the machining class (e.g. sawing, drilling, milling, precision grinding). The following table provides an overview of which tool is suited for what hardnesses:

The wear resistance of the tool is closely associated with the hardness. Stainless steel is usually a very abrasive material and the abrasive tools used must therefore have a high wear resistance. A coating made of titanium nitride, for example, can improve wear resistance. Furthermore, tools should have high heat resistance, since significant heat can be generated when machining stainless steel. Cutting can generate heat as high as approx. 800°C to 1200°C, which is concentrated directly on the tool cutting edge due to the low heat conductivity.

At MISUMI, however, in addition to machining and material-removal tools, you will also find other tools specially adapted for stainless steel, such as tube benders.

Instructions for successful machining

The following instructions can be used as a guide to rule out many errors during machining. Cleanliness is a top priority: particle transfer (airborne rust) from other steels increases the risk of corrosion on otherwise stainless steels. But cleanliness affects not only the working environment, but also the working materials, such as abrasives, themselves. It is also important to allow time for the passive layer to form. At normal ambient temperatures of 20°C, this can take 24-48 hours, and even longer in the presence of moisture. This should be factored in when determining the machining time window.