Manganese steel is the principle material used for crusher wear liners. There is 100 years of experience to draw from which suggests that not all manufacturers have fully utilised the accumulated knowledge. It is important end users understand the basics and feedback to a supplier?s deviations where expectations are not met. The purpose of this paper is to outline some of the basics for manganese steel, eg austenitic, low hardness, work hardens.
Austenitic manganese steel comprises predominantly manganese, carbon and iron in proportions that represent the many commercial grades available. The atomic arrangement of manganese steel is face centred cubic which is a densely packed lattice structure that allows for many slip planes to exist.
The atomic structure explains why austenitic materials are generally soft and ductile and with the correct chemistry will work harden to perform a number of applications. Austenitic manganese steel has outperformed any other material for crusher wear liners for over 100 years.
PROPERTIES OF MANGANESE STEEL
Manganese (Mn) dissolves substitutionally which creates barriers to the movement of dislocations. Carbon dissolves interstitially because of its small size and creates a strong resistance to the movement of dislocations. Carbon has a high solubility in austenite; austenite is non-magnetic.
Austenitic manganese steel has low thermal conductivity. The typical hardness of 14 per cent Mn steel is 200 to 220 BHN in the as-quenched heat treated condition. This does not change significantly with higher Mn contents.
Low hardness also results in good ductility and elongation and means that the yield strength of the material is low. This results in the material deforming at relatively low levels of applied stress.
{{image2-a:l}}In the as-quenched condition, these steels have impact strengths of up to 200 joules, as measured in the Charpy impact test, compared to a plain carbon steel (eg Grade A2) at 30 to 50 joules.
The main reason austenitic manganese steel is used in crusher parts is its ability to work harden when deformed. This results in a hard wear resistant layer forming in the deformed area but backed up by the soft and ductile core. This in effect gives it the mutually exclusive properties of hardness (wear resistance) and ductility (impact strength). Refer to the work-hardened microstructure in Figure 1.
Work hardening can only happen if the material is permanently deformed, therefore, in applications of pure or low stress abrasion, the desired property of higher hardness cannot be achieved, resulting in increased wear.
The degree of work hardening is directly related to the degree of deformation.
Basic principles
All manganese steel grades contain certain elements to give it the basic properties. A number of grades contain additional elements to impart certain modifications to their properties.
In plain carbon steels, austenite only exists at high temperatures, but in austenitic manganese steels, austenite is present at room temperature. This is because manganese is an austenite stabiliser and lowers the minimum temperature at which austenite can exist without transforming to a more stable phase, such as pearlite or martensite.
At around 11 per cent manganese content, the minimum temperature is below room temperature, therefore austenite can exist as a stable phase.
Austenite is also stabilised by nickel, but the price of this material means that it is only used in the higher value alloys like stainless steels for austenite stabilising.
Carbon is also present in all austenitic manganese steels to inhibit the movement of dislocations and so cause work hardening. The rate of work hardening is directly related to the carbon content.
Carbon also slightly increases the yield strength of the material, ie it makes it slightly harder. Carbon is only beneficial in manganese steel if it is in solution in the austenite. If it?s not in solution, it is very detrimental to the mechanical properties and is a frequent cause of failure.
Lean grades
The first group of austenitic manganese steels is the so-called lean grades with manganese contents below nine per cent. These grades have higher initial hardness but much lower rate of work hardening than the higher grades. They also suffer from relatively poor impact properties and are not widely used.
14 per cent manganese steel
This is the original Hadfield?s manganese steel with 11.5 to 15.5 per cent manganese content and 1.0 to 1.4 per cent carbon as alloying elements and silicon as a deoxidant. Other elements may be present in varying amounts as residuals from the melt.
The increased manganese content of these grades result in a slight increase in the rate of work hardening compared to lower manganese grades, but this effect can be achieved at much lower cost by an increase in carbon content. The main benefit from the increase in manganese content is the increase in the yield strength.
23 per cent manganese steel
These are the highest manganese content grades produced by Crushing Equipment Pty Ltd Scaw Metals Foundry Group and range in content from 19 per cent to 24 per cent manganese with carbon contents of 1.2 per cent to 1.4 per cent. These grades have slightly higher initial hardness and yield strength than the lower grades but lower ductility.
Molybdenum grades
Molybdenum is a strong carbide and the carbides formed are distributed in the interior of the grain as globules. This improves the mechanical properties such as impact strength and can also improve the rate of work hardening by creating sites at which dislocations are pinned.
{{image3-a:r}}Because of the type of carbide formed by Molybdenum it is sometimes used in parts that are prone to form carbides, such as thick sectioned castings or parts that will be welded.
Chromium grades
Chromium has been added in the past in the mistaken belief that it will increase wear resistance and improve as-quenched hardness. This has been found to be false and tests done by Scaw Metals Foundry Group as well as the American Society for Testing and Materials (ASTM) show no difference in wear life between chrome bearing grades and standard grades with the same carbon levels.
Chrome is a stronger carbide than iron or manganese and the carbides formed are precipitated mostly along the grain boundary, reducing the weldability and greatly increasing the risk of failure.
Chrome has been popular in manganese steel for jaw liners to prevent the growth of the liner and consequent impingement on the cheek plates, making removal and reversal difficult. This has worked in crushing applications not dependent on optimum mechanical properties to avoid potential failure. Crushing Equipment Pty Ltd promotes liner design as the preferred option to avoid impingement in the crusher and does not supply chrome bearing manganese steel for standard grades unless requested.
USES of MANGANESE STEEL
Austenitic manganese steels find their largest market in the quarry and mining consumable field. Their ability to develop good hardness while having a tough core is utilised in the crushing of ore and aggregate. These alloys also find use in metal to metal sliding abrasion.
Austenitic manganese steel can be used in high stress ambient temperature conditions, with the proviso that the UTS (Ultimate Tensile Strength) of the material is low compared to other available alloys.
METALLURGY CONTROL
The ability of austenitic manganese steels to perform properly depends on correct heat treatment. If the soaking temperature is not high enough the carbides formed while cooling after casting cannot dissolve. These are always on the grain boundaries. If the soaking time is not long enough these same carbides do not dissolve completely.
If temperature and time is correct but the quenching rate is too slow, carbides have time to precipitate and usually do so along the grain boundaries, although in some cases acicular carbides form within the grain on certain atomic planes. This is very detrimental to impact properties. Large, well agitated and temperature controlled water quench tanks are imperative for the manganese steel foundry.
The basic quality check on manganese steels is microscopic analysis. With this procedure the metallurgical quality of the sample can easily be determined.
Hardness tests on as-quenched parts do not indicate the quality of the alloy in any way, but the degree of work hardening can be determined during or after the service life of the part.
{{image4-a:l}}Chemical analysis is the main means of determining the chemical quality of a sample. However, chemical analysis can only tell which grade the sample is, not the metallurgical quality, which can only be determined microscopically.
Although it is not the scope of this report to discuss manufacturing, it is important to note that often failures and high wear rates are not only related to chemistry and metallurgical variables. Manganese steel used for wear liners is a cast material and the inherent high liquid to solid shrinkage and other issues can severely impact wear life if not controlled.
If the crusher operator has any issues with their manganese steel wear liners they must forward the details to their supplier. Improvements and corrective actions will receive appropriate action without the feedback to the manufacturer.
Crusher operators today face a number of issues to ensure they receive the best manganese steel material to meet their crushing requirements. This report is not intended to be comprehensive and answer all the questions that may exist at your operation but it is intended to relay sufficient detail that may prompt further questions and thinking. ?
Lew Dilkes is a qualified metallurgist with Crushing Equipment Pty Ltd.