Part 1: Material properties and trade-offs in material selection
One of the frequently asked questions when selecting wear-resistant materials is: Which material has the highest hardness?
This question can be answered with a numerical value, but that value alone is insufficient to determine which material is suitable for the location being evaluated. Hardness is only one of the properties that need to be considered. When operating conditions also involve impact, temperature, or structural loading, simply increasing hardness may lead to a different failure mode.
At locations primarily subjected to abrasive particles, higher hardness may help the surface resist wear more effectively. Conversely, at locations subjected to direct impact from large lumps of material, an excessively hard material may crack instead of deforming. In such cases, the lining plate may lose its functionality before its thickness has been completely worn away.
Therefore, material selection requires simultaneous consideration of hardness, impact toughness, and tensile properties, while clearly identifying the test method used for each parameter in the technical specification and the level at which it is declared.
1. Higher hardness does not necessarily mean greater suitability
Hardness and toughness are two different properties.
Hardness indicates the ability of a surface to resist scratching or indentation.
Toughness reflects the ability of a material to absorb impact before cracking or fracturing.
Within the same family of wear-resistant steels, as hardness increases, the ability to absorb impact before cracking generally decreases. This is a trade-off that needs to be examined during material selection, rather than simply ranking grades from softer to harder.
At locations primarily subjected to abrasive wear caused by scratching particles, high hardness may be an advantage.
At locations subjected to direct impact from large lumps of material, the toughness requirement increases. If impact resistance is insufficient, the failure mode may shift from progressive thickness loss due to wear to cracking or spalling.
Therefore, in addition to asking “How hard is this material?”, it is necessary to determine “How much hardness does this location require, and how much impact resistance must still be maintained?”
2. Hardness, impact toughness, and tensile properties answer three different questions
These three groups of properties are determined using different test methods and cannot be directly converted into one another.
Knowing the hardness does not mean that the toughness is known.
Similarly, yield strength, tensile strength, or elongation cannot be independently inferred from a hardness value.
Furthermore, the three parameters obtained from a tensile test cannot be used interchangeably.
Three commonly reported parameters in tensile-test results include:
Elongation is a parameter that is easily overlooked when attention is focused only on hardness or yield strength.
The absence of visible cracks after a component has been bent or deformed does not mean that its remaining deformation capacity is unchanged. When the material continues to be used in a load-bearing structure, its mechanical condition needs to be evaluated using appropriate parameters rather than relying solely on surface observation.
3. How to interpret a material specification table
A numerical value in a technical specification is meaningful only when the following are clearly understood:
- which property is being measured
- which test method is used
- the test conditions
- the level at which the manufacturer declares the value
3.1. Do not directly compare values from different hardness scales
Hardness may be specified according to Brinell, Rockwell, or Vickers.
Values from different scales should not be placed side by side and compared directly as though they used the same unit.
Before comparing two materials, it is necessary to check:
- whether the same property is being compared
- whether the same test method is used
- whether the same test conditions apply
If two values come from different scales, they need to be interpreted according to their respective methods rather than treated as two values on the same scale.
3.2. Impact toughness must be considered together with the test temperature
Charpy test results depend not only on the material but also on the test temperature.
The same material tested at two different temperatures may produce two different results.
Therefore, when comparing the toughness of two grades, it is necessary to check not only the Joule values but also whether the test temperatures are the same.
3.3. Guaranteed values, typical values, and converted values have different applications
A parameter may be presented at three levels:
A typical value indicates the range around which a material’s property generally falls, but it does not automatically mean that the manufacturer guarantees that exact value for every product. Pasted text (2)
Therefore, two purposes need to be distinguished:
technical comparison between options and establishment of contractual or acceptance criteria.
A numerical value may be suitable for the first purpose but may not provide sufficient basis for the second.
4. Material hardness must be considered alongside the hardness of the abrasive particles
When evaluating abrasive wear caused by scratching particles, comparing two materials is still insufficient.
It is necessary to determine:
How does the hardness of the material surface compare with that of the particles acting on it?
If the particles are harder than the surface, they can still cut into the surface and remove material.
Therefore, hardness needs to be considered in terms of the relationship:
abrasive particles ↔ material surface
In the field, particle hardness can be assessed approximately using a Mohs hardness test set.
Samples need to be taken from the actual flow causing wear:
- Take a sample directly from the material flow passing through the location.
- Test using different hardness levels in the Mohs test set.
- Determine the hardness range present.
- Record the hardest particle type as well, not only the type accounting for the largest proportion.
Material flow is often non-uniform. Therefore, recording a hardness range is generally more appropriate than using a single value.
For engineering materials, hardness must be interpreted according to the corresponding test method and technical documentation. A Mohs conversion table should not be used to infer a Vickers or Brinell value and then treat it as a direct measurement.
You can learn more about using particle hardness in the material-group screening process in Part 2: Four influencing factors and the material selection sequence.
5. Four errors that can make hardness comparisons meaningless
A conversion table may help indicate the relative hardness level, but it does not replace direct testing when material verification or acceptance is required.
6. The trade-off between hardness and toughness can be observed directly in specification tables
Within the same Hardox® wear-resistant steel product family, the relationship between hardness and impact toughness can be observed through the following reference grades:
The hardness ranges in the table are presented as guaranteed values, while the impact toughness values are typical values, tested according to EN ISO 148-1 using Charpy V-notch specimens. Pasted text (2)
When interpreted in order of increasing hardness, the general trend within this product family indicates decreasing impact toughness.
This explains why the hardest grade available for purchase is not automatically the most suitable grade for every location.
In areas subjected to severe abrasive wear but low impact, high hardness may provide an advantage.
In areas subjected to direct impact from large lumps of material, sufficient toughness needs to be maintained to prevent the failure mode from shifting to cracking or spalling.
6.1. Material technology can improve the balance but does not eliminate the fundamental trade-off
Within the hardness range of approximately 500 HBW, the documentation provides the following comparison:
The point to consider is not merely 45 J versus 37 J.
More importantly, it is the declaration category to which each value belongs.
If a value needs to be included in a technical requirement or contract, the guaranteed value in the technical specification for that specific material grade must be checked.
This case demonstrates that material technology can improve the balance between hardness and toughness within a certain range, but it does not eliminate the requirement to evaluate both properties simultaneously.
7. When abrasion and impact requirements conflict, one material may not be sufficient
Some locations are subjected to extremely hard abrasive particles, resulting in a high surface-hardness requirement.
At the same time, the location is also subjected to severe impact and requires sufficient material toughness to prevent cracking.
This creates two limits:
- lower hardness limit: the minimum hardness required to limit particle cutting of the surface
- upper hardness limit: the maximum hardness that can be used before impact resistance becomes unsuitable for the location
If the lower hardness limit exceeds the upper hardness limit, a single material may not satisfy both requirements.
Approaches requiring consideration include:
- switching to another material group
- combining multiple material groups according to different operating zones
- modifying the flow conditions, where possible, to reduce the severity of the action
The third approach is not a material-selection problem, but it may directly address the cause of the damage.
You can learn more about addressing cases where one material group cannot meet all operating requirements in Part 5: Combining multiple materials at one location: design principles and interface control.
8. Not every material property can be measured directly in the field
Some measurements can be performed in the workshop for preliminary assessment, while other parameters require testing using specialized methods.
A field measurement may be sufficient for screening and preliminary assessment, but it may not necessarily be sufficient for use as an acceptance criterion.
Therefore, before selecting a measurement method, it is necessary to determine what the resulting value will be used for.
9. Responsibilities of the departments
One principle that must be maintained throughout is not to use typical values as guaranteed values.
If a parameter is included in a contract or acceptance criteria, the following need to be verified:
- material grade
- current technical specification
- test method
- test conditions
- declared value category of the parameter
10. Notes when applying this knowledge
First, a specification table describes material properties; it does not directly indicate service life at a specific location. Service life also depends on operating conditions and actual monitoring data.
Second, conversions between hardness scales are only approximate within their applicable ranges. Conversion tables should not be used in place of direct measurements when acceptance testing is required.
Third, parameters must be interpreted from the technical documentation for the specific material grade. Guaranteed values, typical values, and converted values may appear together in the same table; these three groups of values do not have the same evidentiary weight.
Fourth, the standards referenced in this article include EN ISO 6506-1, EN ISO 6508-1, EN ISO 148-1, and EN ISO 6892-1. The Hardox® data used in the comparison are taken from SSAB’s technical documentation and guarantee documentation.
Hardness is an important data point, but it cannot stand alone in the selection decision. A material is suitable only when the required hardness level is considered alongside toughness, mechanical properties, impact conditions, and the actual significance of the values presented in the technical specification.
