Iron-based alloys

Iron-based alloys are a large and widely used class of hard-surface materials, which are characterized by good overall performance, a wide range of performance, and the lowest material prices.
Iron-based alloys can be divided into the following categories according to different metallographic organizations.
(1) Martensitic alloy steel
The main hardening element is Cr, there are Si, Mo, Mn, V, W and other strengthening elements, the total amount of alloying elements does not exceed 10%. The coating organization is low carbon martensite, good machinability, coating hardness HRc30?54, impact toughness is excellent, also has good stress fatigue and hot and cold fatigue resistance. Its material made of metal wire and tubular wire submerged arc overlay welding, welding performance is better, not easy to crack. Typical use is between the metal without lubrication rolling or sliding parts. Such as hot rolling work rolls, support rolls, continuous casting machine rolls, guide rolls, straightening rolls and excavator rolls, etc.
(2) High chromium cast iron
This kind of alloy has high hardness, HRc48?60, and has excellent anti-abrasive wear performance. Wear resistance at <200℃ is second only to tungsten carbide hard surface material, but the price is only 1/3 of tungsten carbide material.
High chromium cast iron contains 2% to 6% carbon and 20% to 35% chromium, the main wear-resistant hard phase Cr7C3 in the weld layer, and the matrix organization is martensite and austenite.
High chromium cast iron using electrode or tubular wire bright arc or submerged arc surfacing, surfacing process due to a large number of carbide precipitation, the weld layer produces cracking release the internal stress in the weld layer, and does not affect its use performance. Mainly used for agricultural machinery, mining, coal grinding machine rollers and other medium or severe abrasive wear parts.
High chromium cast iron type self-melting alloy powder, oxyacetylene flame (or plasma) spray welding, coating hardness (HRc50 or more), for non-strong impact of the abrasive wear parts. [2]
(3) Austenitic manganese steel
High manganese steel hard surface material can withstand high impact and slight to moderate abrasive wear, overlay layer non-magnetic, with high toughness, after welding hardness HRc16 ~ 20, after cold hardening can be increased to HRc4448, with different carbon content and change.
High manganese steel contains Mn12% ~ 15% and Cr, Ni, Mo, austenite organization is stabilized with manganese. Welding without gas welding only with electric welding to reduce the heat-affected zone to obtain a rapid cooling of the weld layer. Improper welding will appear martensite phase and lead to weld layer cell crack.
High manganese steel electrode and bright arc welding wire, for serious intermetallic impact and ore on the metal of the surfacing.
(4) martensitic stainless steel
Such combined metal low carbon high chromium martensitic steel. The main component contains carbon C 0.2% and chromium > 12%. Has good overall mechanical properties, hardness HRc50 or so, strength, toughness are very good, can resist the corrosion of the atmosphere and steam, and the ability to resist hot and cold fatigue. The products are available in metallic wire and tubular wire. It is mainly used in medium impact, medium intermetallic wear and medium abrasive wear applications.
Martensitic stainless steel type spray powder, oxyacetylene flame spraying, coating hardness (HB320 ~ 450), used for shaft, piston, plunger and other wear-resistant parts.
(5) light body steel
This alloy contains low carbon (0 ~ 25%) amount of other alloying elements, the organizational structure of pearlite, impact resistance, low hardness (HRc25 ~ 35), with excellent weldability, suitable for overlay welding, mainly used to restore the size of mechanical equipment parts. Such as rolling, sliding or impact load of heavy machinery and equipment rotating shaft, rollers and other parts. Another important use is as overlay welding transition layer.

The main role of alloying elements
1. The effect on the mechanical properties of steel
(1) Solid solution strengthening When alloying elements are dissolved in ferrite, they have solid solution strengthening effect. When the lattice type and atomic diameter of alloying elements are different or different from α-Fe, the strengthening effect on ferrite is more obvious, and vice versa, the strengthening effect is weaker. Therefore, different elements contribute differently to the increase in hardness of ferrite with increasing content, as shown in the figure. Alloying elements make ferrite solid solution strengthening at the same time, especially in the case of higher content of alloying elements, often due to the serious ferrite lattice distortion, and make toughness and plasticity decline, as shown in the figure. Therefore, in order to make the steel has a high overall performance, the steel added to the alloying elements, should be a plurality of small amounts, rather than an element to add more the better.

(2) dispersion strengthening alloying elements and carbon to form carbide, and with fine plasmas distributed in the solid solution matrix, can play a dispersion strengthening effect, so that the steel hardness and strength to further improve.
In addition to the above strengthening effect, when the number of carbides in the steel, will significantly improve the hardness and wear resistance of steel. Some carbide solution point is high, high stability, put can improve the thermal strength of steel.
2. The effect on Fe-Fe3C phase diagram
Some alloying elements, such as face-centered cubic lattice of nickel, manganese, copper and non-metallic elements nitrogen, can make Fe-Fe3C phase diagram in the γ zone expansion (Figure 8-2 (o)), while some other elements, such as body-centered cubic lattice of chromium, molybdenum, titanium, etc., can make the y zone shrink Figure s-2Cb))
The influence of alloying elements on the P-region leads to the following changes.
(1) change in the critical point
Elements that expand the γ-zone will lower the A1 and A3 temperatures, and elements that narrow the γ-zone will increase the A3 and A1 temperatures. Therefore, the heat treatment heating temperature of alloy steel, will be correspondingly lower or higher, different from the heating temperature of carbon steel. Expand the γ zone of the elements, under certain conditions, can make the γ zone expanded to room temperature, and thus can be obtained single-phase austenitic steel. This is the basis of austenitic stainless steel will be addressed in this chapter. Narrowing the γ zone of the elements, under certain conditions, can make the austenite phase zone disappears and only the ferrite phase zone exists, and thus can be obtained single-phase ferritic steel. This is also the basis for the industrial use of ferritic stainless steel and heat-resistant steel.
(2) S-point left shift
Alloying elements make the S point left shift, so the alloy steel co-precipitation body carbon content is less than the carbon steel co-precipitation body 0.77% carbon content. For example, when the steel contains 13% Cr, the carbon content of the eutectic is only 0.3%.
(3) F point left shift
E point of iron-carbon alloy corresponding carbon content of 2.11%, when the steel contains alloy elements, E point corresponding carbon content of less than 2.11%, so that the alloy steel in the lower carbon content of eutectic Leydenite.
3. Refine austenite grain
When the alloying elements form insoluble compounds (TiC, NbC, Al2O3, AlN, etc.), these compounds exist on the austenite grain boundaries, mechanically prevent the growth of austenite grains, so that the organization of austenite after cooling transformation is fine, and thus play a role in the fine crystal strengthening.
4、Improve the hardenability of steel
Austenite dissolved with alloying elements, where the atomic diffusion ability of alloying elements is small, but also reduce the diffusion ability of iron and carbon atoms in austenite, so that the stability of austenite increased, not easy to pearlite transformation. Reflected in the C curve, so that the C curve right shift (Co elements exception) and make the quenching critical cooling rate is reduced, improve hardenability. Therefore, alloyed steel not only improves the overall cross-sectional mechanical properties, but also reduces the risk of quenching deformation and cracking. However, the alloying element shifts the C curve to the right while lowering the Ms point, which increases the amount of residual austenite after quenching and is detrimental to improving hardness and wear resistance.