HEAT TREATMENT OF BLADE STEEL

By Marie Nel

WHAT MAKES A STEEL? BLADE STEEL COMPOSITION & PROPERTIES

Element Properties

Carbon (C) -Increases resistance to wear & hardness

-Improves edge retention & tensile strength

-Single most important hardening element

Chromium (Cr) -Improves hardness, toughness & tensile strength.

-Significantly improves corrosion resistance.

Cobalt (Co) -Enables higher temps when quenching during heat treat.

-Works well with other elements to boost properties in complex steels.

-Increases strength and hardness

Copper (Cu) -Increases wear resistance as well as corrosion resistance

Manganese (Mn) -Allows higher levels of hardening.

-Improves wear resistance & tensile strength

Element Properties

Molybdenum

(Mo)

-Improves ease of machining.

-Increases harden-ability, toughness & strength.

-Prevents brittleness

Nickel (Ni) -Adds corrosion resistance, hardness & strength.

Phosphorous (P) -Increases machinability, hardness and strength.

-In high enough concentration, causes brittleness.

Silicon (Si) -Increases tensile & yield strength.

Sulphur (S) -Improves machinability but lowers toughness.

Tungsten (W) -Increases strength, hardness & toughness.

Vanadium (V) -Increases strength, hardness and shock (impact) resistance.

-Retards growth.

KMTs KNIFE STEEL CHART - COMPOSITION

Steel C Carbon

N Nitrogen

Cr Chrome

Co Cobalt

Mn Manganese

Mo Molibium

P Phosphor

S Sulphur

Si Silicon

T Tungsten

V Vanadium

Conventional Stainless Steel

N690 L4528

1.08

17.30

1.50

0.40

1.10

0.40

0.10

12C27 0.60 0.40 <0.025 <0.01 0.40

14C28N 0.62 0.11 14.00

Powder Steel

M390 1.90 20.0 0.30 1.00 0.70 0.60 4.00

CHOOSING THE RIGHT KNIFE STEEL

When choosing the steel and its hardness for a knife, it is important to bear in mind the application for which the knife will be used. The recommendations for selection of steel grade and hardening program are based on the combined requirements for edge performance, toughness and corrosion resistance.

Edge performance – consists of 3 elements: sharpness, edge stability and wear resistance

Sharpness

The ability of the steel to support a keen edge with razor sharpness. It also means that the knife will be easy to resharpen. This is important for all knives.

Edge stability

The ability for the knife edge to withstand edge rolling and edge micro-chipping. Rolled edges and micro-chipped edges are the most common reasons for resharpening. This is important for all knives.

Wear resistance

The ability for the edge to resist abrasive wear. This is usually secondary to edge stability issues, such as micro-chipping or edge rolling.

CHOOSING THE RIGHT KNIFE STEEL

Toughness

Toughness is the resistance of the knife to cracking. Cracks always start at a weak point in the steel, such as an inclusion or a large primary carbide. So toughness is enhanced by a homogeneous structure that is free from impurities and large carbides. A fine-carbide steel grade will always have higher toughness than a coarse-carbide grade with a given hardness. Toughness is vital for professional and military knives.

Corrosion resistance

Corrosion resistance should be selected to suit the application. Since high corrosion resistance involves sacrifices in edge performance, the best approach is to have corrosion resistance that is 'good enough' for the selected type of knife. An everyday carry knife and a fishing knife will make very different demands on corrosion resistance.

KIND OF KNIVES FOR SPECIFIC STEELS (EXAMPLES)

Always refer to the Manufactures Specification, the information reflected are merely an overview

TYPE OF KNIVES 12C27 14C28N N690

Chef’s knives x x

Dive knives x

Fisherman’s knives x x x

Folding knives x x x

Hunting knives x x

Kitchen knives x

DEFINITIONS: HARDENING AND TEMPERING

The definition of heat treatment: a combination of heating and cooling operations, timed and applied to a metal in a solid state in a way that will produce desired properties. All basic heat treating processes involve the transformation or decomposition of austenite. The nature and appearance of these transformation products determine the physical and mechanical properties of any given steel.

The basic purpose of hardening is to produce a fully martensite structure, and the minimum cooling rate (°C per second) that will avoid the formation of any of the softer products of transformation. The critical cooling rate, determined by chemical composition and austenite grain size, is an important property of a steel since it indicates how fast a steel must be cooled in order to form only martensite.

Process: Hardening is a way of making the knife steel harder. By first heating the knife steel to between 1050 and 1090°C and then quickly cooling (quenching) it, the knife steel will become much harder, but also more brittle.

To reduce the brittleness, the material is tempered, usually by heating it to 175–350°C for 2 hours, which results in a hardness of 53–63 HRC and a good balance between sharpness retention, grindability and toughness.

WHAT HAPPENS INSIDE THE MATERIAL WHEN HARDENING AND TEMPERING OF STEEL

a) Original structure, coarse-grained ferrite and pearlite

b) Just above the lower critical A1 line (723°C): pearlite has transformed to small grains of austenite, ferrite unchanged. Cooling from this temperature will not refine the grain.

c) Above the upper critical A3 line (884 °C): only fine grained austenite. The proper annealing temperature are ± 10 °C above the A3 line.

d) After furnace cooling to room temperature, fine- grained ferrite and small pearlite areas. The grain size are refined, but the structure did not changed.

WHAT HAPPENS INSIDE THE MATERIAL WHEN HARDENING AND TEMPERING OF STEEL

Full Annealing

Refinement of the grain size will occur ±10°C above the upper- critical temperature A3 line , which under slow or moderate cooling rates, the C-atoms are able to diffuse out of the austenite structure.

With a still further increase in cooling rate, insufficient time is allowed for the carbon to diffuse out of the solution, and although some movement of the Fe-atoms takes place, with C trapped in the solution. The resultant structure, called martensite, is a supersaturated solid solution of C trapped in a structure where 2 dimensions of the unit cell are equal, but the 3rd is slightly expanded because of the trapped C. This highly distorted lattice structure is the prime reason for the high hardness of materials.

WHAT HAPPENS INSIDE THE MATERIAL WHEN HARDENING AND TEMPERING OF STEEL

After drastic cooling (quenching), martensite appears microscopically as a white needlelike structure sometimes described as a pile of straw. The transformation proceeds only during cooling and ceases if cooling is interrupted. Therefore, the transformation depends only upon the decrease in temperature and it is independent of time. The amount of martensite formed with decreasing temperature is not linear. The number of martensite needles produced at first is small, then the number increases, and finally, near the end, it decreases again.

Optimum hardening produces an unstructured matrix of tempered martensite with very small, uniformly distributed carbides, and a certain amount of residual austenite. The content of retained austenite should be between 5 and 15%.

If the straightness or flatness of the knife blades is found to need adjustment after quenching, this is best done before the material is tempered, at least before it has had time to cool to room temperature.

WHAT HAPPENS INSIDE THE MATERIAL WHEN HARDENING AND TEMPERING OF STEEL

Deep freezing of knife steel

Deep-freezing is used if cooling to room temperature does not produce sufficient hardness, and involves cooling down the knife blades to a temperature in the range -20°C to -150°C before they are tempered.

The simplest way of deep-freezing the knife blades is to place them in a freezer or immerse them in dry ice. The knife blade is then left to 'thaw' to room temperature, and is then tempered in the usual way.

Deep-freezing increases the hardness by 1–3 HRC, but reduces the toughness slightly. For most applications, hardness between 57 and 60 HRC provides a good balance between edge stability, toughness and grindability.

WHAT HAPPENS INSIDE THE MATERIAL WHEN HARDENING AND TEMPERING OF STEEL

Annealing should never be a final heat treatment as the network is much harder, but also brittle which tends to be a plane of weakness. The presence of a thick and hard grain boundary will also result in poor machinability.

Tempering

In the as-quenched martensite condition, the steel is to brittle for most applications. The formation of austenite leaves high residual stresses in the steel. Therefore, hardening is almost always followed by tempering, which consists in heating the steel to some temperature below the lower critical temperature. The purpose of tempering is to relieve residual stresses and to improve the ductility and toughness of the steel.

Residual stresses are relieved to a large extend when the tempering temperature reaches 204°C, and by 480 °C they are almost completely gone.

Tempering should be carried out within a reasonable time after hardening, preferably within an hour or so. It is of vital importance that the blade should be allowed to cool to room temperature before tempering is started. The transformation to martensite will otherwise be interrupted and the hardening results may be impaired.

A higher tempering temperature will yield a somewhat softer material with higher toughness, whereas a lower tempering temperature will produce a harder and somewhat more brittle material, as shown by the figure below.

WHAT HAPPENS INSIDE THE MATERIAL WHEN HARDENING AND TEMPERING OF STEEL

A camping knife or a survival knife, for example, may be tempered at 350°C so that it will be able to withstand rough handling without breaking. On the other hand, if the knife is expected to keep a sharp edge, it can instead be tempered at 175°C for maximum hardness.

Tempering temperatures below 175°C should be used only in exceptional cases, when extreme demands are made on high hardness, since very low tempering temperatures will result in a very brittle material. Similarly, tempering temperatures above 350°C should be avoided, since this could give rise to brittleness and reduced corrosion resistance. Note that if the tempered blade is exposed to temperatures above the tempering temperature (e.g. during grinding), the properties of the knife will be impaired.

Correctly performed hardening will result in a good balance between hardness, toughness and corrosion resistance of the finished knife blade.

KMTs KNIFE STEEL CHART - HARDENING

General soaking time rule: 1 hour per inch thickness

STEEL

HRC

RANGE

ANNEAL RANGE

STRESS RELIEVE

HARDENING

RANGE

SUB-0

QUENCH

TEMPER

RANGE TIME TIMES

Conventional Stainless Steel

N690 L4528

60-58 800-850 600 1030-1080 100-200 1h X2

12C27 61-54 850 650 1070-1090 -70 (-20) 150-400 2h X1

14C28N 62-55 850 650 1045-1085 -70 (-20) 175-350 2h x1

Powder Steel

M390 64-60 1050 650-700 1070 150-300 2h x2

REFERENCES

• Avner: Introduction to physical metallurgy, Second Edition

• KMTs Knife Steel Chart – Composition

• KMTs Knife Steel Chart – Hardening

• Sandvik Materials: Knife Steel