Knife hardening: carbon steel and alloy steel

Knife blades can be made of high carbon steel, and from low-alloy. Usually knife blades are hardened. The hardening result of these - very different - steels can also be quite different.. Below, on the example of two steels, the features of the hardening of knife blades from carbon and alloy steel.

Steel knife hardening

Two steel grades - American 1086 and 5150 (our analogues – become 85 and 50X) - were forged into identical blanks for knives, as shown in the picture 1.

zakalka-nozhaPicture 1 - Two possible configurations for the distribution of martensite and pearlite
in hardened steel blades:
and) expected distribution of martensite (M);
b) actual distribution of martensite

The blade width of the knife was 25 mm, and the thickness varied from 6 mm to 0,3 mm. Samples with fine and coarse grain were prepared for each steel.. quenching was carried out in an oil tank at room temperature with oil stirring. Then the blanks were cut in half, polished, poisoned and examined under a microscope.

Three samples of four had 100% martensite: both fine-grained and coarse-grained from alloy steel 5160 and a coarse carbon steel sample 1086.

Microstructure of the knife blade steel

Fine grain steel specimen structure 1086 turned out to be a mixture of martensite and finely dispersed pearlite. In this case, the distribution of martensite over the cross section turned out to be unexpected. Since the cooling rate had to be highest on the blade surface, then, seemingly, martensite should have formed along the entire surface. However, martensite was found only in the narrow part of the blade and slightly in the corners on the thick edge. (picture 1, b).

That fact, that no martensite was formed, as was expected, along the surface, means that, that this tapered blade, when cooled in oil, the cooling rate in the center of the blade is almost the same as on the surface of the thickened part 6 mm. This effect is known for thin samples and it is associated with film boiling on its surface..

Knife blade steel hardness

All hardened specimens were then examined by measuring the hardness HRC at intervals 0,06 mm across the blade from thin to thick edge. The hardness across the blade thickness is shown in the figure 2.

tverdost-nozhaPicture 2 - HRC hardness by blade thickness

Microhardness of steel along the blade section shows, However, that the hardness remains constant across the thickness of the blade from 2 mm to the thin edge of the blade, where the thickness is approximately 0,8 mm.

Effect of grain size on steel hardenability

  • Results for steel blade 1086 show a strong effect of grain size.
  • In coarse-grained steel (grain score 11) the blade is fully martensitic with HRC = 65 throughout the thickness.
  • In a fine-grained steel blade 1086 (grain score 15) 100%-martensite was able to form only up to a thickness of about 2 mm.
  • With a thickness 4 mm there is no martensite at all, and the structure of the steel consists of bainite or pearlite. Steel 5160 feels much less the effect of grain size. A small loss of martensite is observed only at thicknesses greater than 4 mm.

This example demonstrates the advantages of low alloy steels over conventional carbon steels.. Fine grain gives them high viscosity, and good hardenability - a greater depth of hardening - much deeper, than carbon steels.

Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007