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ISO P Steel


Definition

  • ​Steel is the largest workpiece material group in the metal cutting area.
  • Steels can be non-hardened, or hardened and tempered with a common hardness up to 400 HB. Steel with a hardness above approx. 48 HRC and up to 62-65 HRC belong to ISO H.
  • Steel is an alloy with iron as the major component (Fe-based).
  • Unalloyed steels have a carbon content lower than 0.8%, and are composed solely of iron (Fe), with no other alloying elements.
  • Alloyed steels have a carbon content that is lower than 1.7 % and alloying elements such as Ni, Cr, Mo, V and W.
  • Low alloyed steels have alloying elements less that 5%.
  • High alloyed steels have more than 5% alloying elements.

 

Machinability in general

  • ​The machinability of steel differs, depending on alloying elements, heat treatment and manufacturing process (forged, rolled, cast, etc.).
  • In general, chip control is relatively easy and smooth.
  • Low carbon steels produce longer chips that are sticky and require sharp cutting edges.
  • Specific cutting force kc1: 1400-3100 N/mm².
  • Cutting forces, and thus the power required to machine them, remain within a limited range.

 

Alloying elements
C influences hardness (higher content increases abrasive wear). Low carbon content <0.2%, increases the adhesive wear, which will lead to built-up edge and bad chip breaking.
Cr, Mo, W, V, Ti, Nb (carbide formers) – increase abrasive wear.
O has a large influence on machinability: it forms non-metallic,
oxidic and abrasive inclusions.
Al, Ti, V, Nb are used as fine-grained-treatment of steel ; they make the steel tougher and more difficult to machine.
P, C, N in the ferrite, lowers the ductility, which increases adhesive wear.

Positive effect
Pb in free machining steel (with low melting point) reduces friction between chip and insert, lowers wear and improves chip breaking.
Ca, Mn (+S) form soft lubricating sulphides. High S-content improves machinability and chip breaking.
Sulphur (S) has a beneficial effect on machinability. Small differences, such as those between 0.001% and 0.003% can have substantial effects on machinability. This effect is used in free machining steels. Sulphur content of around 0.25% is typical. Sulphur forms soft manganese sulfide (MnS) inclusions that will form a lubricating layer between the chip and the cutting edge. MnS will also improve the chip breakage. Lead (Pb) has a similar effect and is often used in combination with S in free machining steels at levels of around 0.25%.

Both positive and negative
Si, Al, Ca form oxide inclusions that increase wear. Inclusions in the steel have an important influence on the machinability, even though they represent very small percentages of the total composition. This influence can be both negative and positive. For example, aluminium (Al) is used to deoxidize the iron melt. However, aluminium forms hard abrasive alumina (Al2O3), which has a detrimental effect on machinability (compare the alumina-coating on an
insert). This negative effect can, however, be counteracted by adding Calcium (Ca), which will form a soft shell around the abrasive particles.

  • Cast steel has a rough surface structure, which can include sand and slag, and places a high demand on the toughness on the cutting edge.
  • Rolled steel exhibits a fairly large grain size, which makes the structure uneven, causing variations in the cutting forces.
  • Forged steel has a smaller grain size and is more uniform in structure, which generates fewer problems when cut.
​     

 

MC codes for steels

​Steels are, from a machinability point of view, classified into unalloyed, low alloyed, high alloyed and sintered steels.

 

Unalloyed steel – P 1.1-1.5

​Definition

In unalloyed steels, the carbon content is usually only 0.8%, while alloyed steels have additional alloying elements. The hardness varies from 90 up to 350HB. A higher carbon content (>0.2%) enables hardening of the material.

 

​Common components

Predominant uses include: constructional steel, structural steel, deep drawn and stamped products, pressure vessel steel, and a variety of cast steels. General uses include: axles, shafts, tubes, forgings and welded constructions (C<0.25%).
 
 

 

​Machinability

Difficulties in chip breaking and smearing tendencies (built-up edge) require special attention in low carbon steels (< 0.25%). High cutting speeds and sharp edges and/ or geometries, with a positive rake face and thin coated grades, will decrease the smearing tendencies. In turning, it is recommended that the depth of cut remains close to or bigger than the nose radius to improve chip breaking. In general, the machinability is very good for hardened steels, however, they tend to generate relatively large flank wear on the cutting edges.

 

Low alloyed steel – P 2.1-2.6

​Definition

Low alloyed steels are the most common materials currently available in metal cutting. The group includes both soft and hardened materials (up to 50 HRc).

 

​Common components

Mo- and Cr-alloyed pressure vessel steels are used for higher temperatures. General uses include: axles, shafts, structural steels, tubes and forgings. Examples of components for the automotive industry are: con rods, cam shafts, cv-joints, wheel hubs, steering pinions.

 

​Machinability

Machinability for low alloyed steels depends on the alloy content and heat treatment (hardness). For all materials in the group, the most common wear mechanisms are crater and flank wear.
 
Hardened materials give higher heat in the cutting zone and can result in plastic deformation of the cutting edge.
 

 

High alloyed steel – P 3.0-3.2

​Definition

High alloyed steels include carbon steels with a total alloy content over 5%. The group includes both soft and hardened materials (up to 50 HRc).

 

​Common components

Typical uses of these steels include: machine tool parts, dies, hydraulic components, cylinders and cutting tools (HSS).

 

​Machinability

In general, machinability decreases at higher alloy contents and hardness. For example, at 12-15% alloying elements and hardness up to 450 HB, the cutting edge needs good heatresistance to withstand plastic deformation.

 

 
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