Definition
- An alloy with the element iron (Fe) as the major constituent.
- Has a chrome content which is higher than 12%
- Has a generally low carbon content (C ≤ 0.05 %).
- Various additions of Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Niobium (Nb) and Titanium (Ti), supply different characteristics, such as resistance towards corrosion and strength at high temperatures.
- Chrome combines with oxygen (O) to create a passivating layer of Cr2O3 on the surface of the steel, which provides a non-corrosive property to the material.
Machinability in general
The machinability of stainless steels differs depending on alloying elements, heat treatment and manufacturing processes (forged, cast, etc.) In general, machinability decreases with a higher alloy content, but free-machining or machinability improved materials are available in all groups of stainless steels.
- Long-chipping material.
- Chip control is fair in ferritic/martensitic materials, becoming more complex in the austenitic and duplex types.
- Specific cutting force: 1800-2850 N/mm².
- Machining creates high cuttting forces, built-up edge, heat and work-hardened surfaces.
- Higher nitrogen (N) content austenitic structure, it increases strength and provides some resistance against corrosion, but lowers machinability, while the deformation hardening increases.
- Additions of Sulphur (S) are used to improve machinability.
- High C-content (>0.2%) provides relatively large flank wear.
- Mo and N decrease machinability, however, they provide resistance to acid attacks and contribute to high temperature strength.
- SANMAC (Sandvik trade name) is a material in which machinability is improved by optimizing the volume share of sulphides and oxides without sacryficing corrosion resistance.
MC codes for stainless steel
Identification of material group
The microstructure that a stainless steel attains depends primarily on its chemical composition, in which the main alloy components Chromium (Cr), and Nickel (Ni) are most important, see diagram. In reality, the variation can be wide, due to the influence of other alloy components that strive to stabilize either the austenite or the ferrite. The structure can also be modified by heat treatment or, in some cases, by cold working. Precipitation hardening ferritic or austenitic stainless steel have an increased tensile strength.
Ferritic and martensitic stainless steel – P5.0-5.1
Definition
From a machinability point of view, ferritic and martensitic stainless steels are classified as ISO P. Normal Cr-content is 12-18%. Only small additions of other alloying elements are present.
Martensitic stainless steels have relatively high carbon content, which make them hardenable. Ferritic steels have magnetic properties. Weldability is low for both ferritic and martensitic and medium to low resistance against corrosion, which increases with a larger Cr-content.
Common components
Often used in applications that place a limited demand on corrosion resistance. The ferritic material is relatively low cost due to the limited Ni-content. Examples of applications are: shafts for pumps, turbines steam and water turbines, nuts, bolts, hot water heaters, pulp and food processing industries due to lower requirements on corrosion resistance.
Martensitic steels can be hardened and are used for the edges in cutlery steel, razor blades, surgical instruments, etc.
Machinability
In general, the machinability is good and very similar to low alloyed steels, therefore it is classified as an ISO P material. High carbon content (>0.2%) enables hardening of the material. Machining will create flank and crater wear with some built-up edge. ISO P grades and geometries work well.
Austenitic and super-austenitic stainless steel – M1.0-2.0
Definition
Austenitic steels are the primary group of stainless steels; the most common composition is 18% Cr and 8% Ni (e.g.18/8-steels, type 304). A steel with better resistance to corrosion is created by adding 2-3% molybdenum, which is often called “acid-proof steel”: (type 316). The MC group also includes super-austenitic stainless steels with a Ni-content over 20%. The austenitic precipitation hardening steels (PH) -steels have an austenitic structure in the solution heat treated condition and a Cr-content of >16% and a Ni-content of >7%, with approx. 1% Aluminum (Al). Typical precipitation hardened steels include 15-5 PH, 17-4 PH, 13-8MPH and 17/7 PH steel, as well as Nitronic 50, Nitronic 60, etc.
Common components
Used in components where good resistance against corrosion is required.
Very good weldability and good properties at high temperatures. Applications include: the chemical, pulp and food processing industries, exhaust manifolds for airplanes. Good mechanical properties are improved by cold working.
Machinability
Work hardening produces hard surfaces and hard chips , which in turn lead to notch wear. It also creates adhesion and produces built-up edge (BUE). It has a relative machinability of 60%. The hardening condition can tear coating and substrate material from the edge, resulting in chipping and bad surface finish. Austenite produces tough, long, continuous chips, which are difficult to break. Adding S improves machinability, but results in lowered resistance to corrosion.
Use sharp edges with a positive geometry. Cut under the work hardened layer. Keep cutting depth constant. Generates a lot of heat when machined.
Duplex stainless steel – M 3.41-3.42
Definition
By adding Ni to a ferritic stainless Cr-based steel, a mixed base structure/matrix will be formed, containing both ferrite and austenite. This is called a duplex stainless steel. Duplex materials have a high tensile strength and maintain a very high corrosion resistance. Designations, such as super-duplex and hyper-duplex indicates higher content of alloying elements and even better corrosion resistance. A Cr-content between 18 and 28%, and a Ni-content between 4 and 7% are common in the duplex steels and will produce an ferritic share of 25-80%. The ferrite and austenite phase are usually present at room temperature at 50-50% respectively. Typical SANDVIK brand names are SAF 2205, SAF 2507.
Common components
Used in machines for the chemical, food, construction, medical, cellulose and papermaking industries and in processes that include acids or chlorine. Often used for equipment related to off-shore oil and gas industry.
Machinability
Relative machinability is generally poor, 30%, due to high yield point and high tensile strength. Higher content of ferrite, above 60%, improves machinability. Machining produces strong chips, which can cause chip hammering, and create high cutting forces. Generates a lot of heat during cutting, which can cause plastic deformation and severe crater wear.
Small entering angles are preferable to avoid notch wear and burr formation. Stabilty in tool clamping and workpiece fixing is essential.