The influence of alloying elements, as described in literature, might not be valid for steel in the context of
blades, in particular for pattern-welded steel. Special care should be used for cutting performance and endurance, since these depend mainly on the angle of the cutting edge as well as on the size of the carbides. Cutting tools cover a wide field of operation of which blades represent only a fraction. Even among blades a differentiation of the purpose is essential
Such a differentiation is also required for the weldability of steels. In general good weldability concerns transformations
in the microstructure of the heat-influence zone and the resulting side effects like welding fissures and stresses, grain growth, etc.
forge-welding cannot be compared with the modern welding techniques since these are normally not performed at temperatures above Ac3 or before the final heat treatment, eliminating the influence of the welding temperature.
Chrome reduces the cooling speed required for a martensitic hardening, as a result of which the harden- and annealability
increase. Chrome narrows the area of gamma-crystal in the iron-carbon equilibrium diagram.
With increasing chrome content the forge-weldability decreases. Above 1-2% mokume-gane techniques have to be used instead, making chrome-containing steels only to a limited extent suitable for pattern welding. At about 12% Cr steel become stainless. Chrome-containing steels are bright after etching.
Carbon is the most essential alloying element of steel. It is substantially responsible for the hardening of steel. The influence of carbon is discussed in more detail in the section "fundamentals".
Manganese expands the gamma-area significantly. The cooling rates required for hardening are strongly reduced, thus increasing the hardness penetration depth. Smaller sections, like in blades, will air-harden. Steels containing above 12% manganese are austenitic
at room temperatures. A content of 4 to 10% manganese will cause steels to harden martensitic even when slow-cooling. Due to the
poor workability, these steels are normally not produced. Manganese acts deoxidizing and strongly sulfur-binding. Normally the etched surface of manganese-steels is dark.
Molybdenum decreases required cooling rates and supports the formation of a fine microstructure, increasing the weldability. The gamma-section is narrowed, forgeability decreases with increasing molybdenum content. It is a strong carbide creator, increasing the mechanical strength and yield point. Molybdenum is often used in high-speed steel to improve the wear resistance.
Nitrogen forms nitrides, giving steel a hard surface layer when nitriting. Nitrogen atoms are a replacement for carbon in steel. The nitrogen atom is slightly smaller than the carbon atom, causing less deformation of the martensitic primary crystal-cell. The risk of aging due to segregation is increased, pronouncing the effect of blue-brittleness as well.
Titanium is a strong deoxidizer, is a strong nitrogen and carbon binder, builds up sulfides and narrows the gamma-section
significantly. Titanium acts as a grain-refiner but tends to banded segregations at higher contents.
Vanadium narrows the gamma section and is a very strong carbon binder. Vanadium in smaller quantities replaces iron as a substitution element
and is grain-refining. With increasing vanadium content or incorrect heat treatment, the present vanadium will act as a very strong
carbon binder, the surrounding area can be depleted of carbon and might not be hardenable any more. Vanadium-carbides are in general
extremely large (50-70 Ám) and very hard (about 2800 HV), thus increasing the resistance to wear and elevated temperatures, making vanadium an essential alloying element with high speed steels.
Tungsten-alloyed steels are increasingly prone to red-shortness (tending to crack during forging at higher temperatures) and show an increased oxidation. They can be forge-welded with caution. Tungsten is an extremely strong carbide-binder, forming very hard and small carbides, thus hindering grain growth and improving the toughness, also at elevated temperatures. Tungsten-alloyed steels are mainly used for high speed steels and elevated temperature speed steels. They are also applied to tools requiring a fine cutting edge.
Steel-Harming Alloying Elements
Arsenic strongly promotes segregations whose elimination by annealing is difficult to impossible. Toughness, weldability and tempering brittleness are negatively influenced. The gamma-section is cut-off, the melting temperature lowered. Arsenic steel has
been used as a solder when forge-welding. The influence on the gamma-section leads to carbon being pushed away from the arsenic zones.
Phosphorus-alloyed steels tend to primary segregations and, due to the gamma-section cut-off, secondary segregations. The diffusion
speed of phosphorus is low; hence, these segregations are difficult to be eliminated in the alpha- and gamma-crystal. The segregations
increase the tempering- and cold-brittleness, the steels become red-short and tent to brittle failure.
Oxygen decreases the impact strength and the ageing brittleness. Oxygen entering steel while forging causes red-shortness, damaging the steel permanently.
Sulfur leads to extremely many segregations and consequently causes red-shortness and decreases the welding point.
The addition of manganese binds the sulfur to manganese-sulfide. Sulfur from the forge coals diffuses into the surface of the steel bar complicating the forge-weld by causing local melting at welding temperature. Free-cutting steels are often sulfur enriched (up to 0.4%) to improve the machinability, leading to short breaking chips.
Silicon gets into the steel while smelting. Steel containing more than 0.4% Si is called silicon steel. Silicon
acts deoxidizing and eases graphite segregation. The gamma-section is narrowed while increasing the elastic limit,
making silicon a well-suited alloying element for spring steel.
© 2005 G.v.Tardy