10 Questions You Should to Know about High-Performance Alloy Tool Steel

Product material data > 20 questions about steel

20 questions about steel

1. What is steel?

Steel is a metallic alloy made primarily from iron and carbon, with small amounts of other elements such as manganese, nickel and boron. The properties of steel can be modified through various heat treatment processes and the addition of different alloying elements. Steel is known for its strength, durability and versatility, and is used in a wide range of applications, including; construction, transportation, machinery and appliances. 

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2. How is steel made step by step?

The raw material for steel is iron ore and/or recycled steel scrap. The molten iron, which is produced in a blast furnace (BF) or electric arc furnace (EAF), is purified in a converter. The required alloying elements are added in the secondary metallurgy. Finally, the steel is cast into slabs, which are then rolled and heat treated. Before delivery, the material can be leveled, blasted, galvanized, metal and color coated. 

3. What is the density of steel?

The density of steel is approx. 7.85 g/cm3 or 490 lb/ft3. 

4. How do the material properties change after forming?

When deforming a piece of steel, the resistance against further deformation increases. This is strain or deformation hardening. Yield strength and hardness will increase, while some ductility is lost.

5. What is the tensile test for steel?

Tensile testing is one of the best-known tests for steel and can be defined by standard EN . Controlled displacement is applied on a standard specimen. The relation between load and elongation is recorded and used to determine a number of material properties, e.g. yield strength and tensile strength. 

6. What is the yield strength and the tensile strength of steel?

Yield strength is the maximum stress level that steel can withstand before plastic deformation occurs. Above this stress level, steel will no longer return to its initial dimensions or shape. Ultimate tensile strength is the stress level where the load is at a maximum during tensile testing and using the original cross section area.

7. What is the stress-strain curve for steel?

The stress-strain curve for steel shows the relationship between applied load and elongation during tensile testing. It is used to determine mechanical properties of the material, including modulus of elasticity, yield strength, and ultimate tensile strength. The load values are transformed into stress values and the elongation is transformed into strain, resulting in a curve that provides important information on how the material behaves under different loading conditions.

8. What is yield ratio in steel?

The yield ratio is yield strength (Rp0.2, fy, Reh) divided by tensile strength (Rm, fu). This value can give an indication of the ductility of the steel, the strain hardenability and an intuitive safety margin against plastic instability or failure. 

9. What is the difference between MPa and ksi?

MPa (mega pascal) is the metric unit and ksi (kilopound per square inch) is the imperial unit for stress (force per unit area). 1 MPa ≈ 0.145 ksi or 1 ksi ≈ 6.89 MPa.

10. What is the Young's modulus of steel?

Young’s modulus, or modulus of elasticity, is the slope of the stress-strain curve in the initial elastic regime. It is defined by the initial linear part of a stress-strain curve. Young’s modulus of steel is approximately 200 GPa or 29 000 ksi at room temperature.

11. What is the difference between steel stiffness and strength?

Stiffness is the ability of the material to resist elastic deformation, which is closely related to the modulus of elasticity. Strength is the ability to resist plastic deformation or failure. The stiffness and load carrying capacity of a real structure (e.g. a car body) are also determined by its geometry. 

13. What are the thermal properties of steel?

Thermal Conductivity, Thermal Expansion Coefficient and Specific Heat are some examples of the thermal properties of steels. They vary with the chemical composition of the steel and with the temperature.

17. What is Poisson's ratio for steel?

Poisson's ratio (ν) measures the deformation in the material in a direction perpendicular to the direction of the applied force. Poisson’s ratio for SSAB steels is approximately 0.3. 

18. Is steel magnetic?

Yes, all SSAB steels are ferritic and thus magnetic. Some plates might contain residual magnetism. 

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19. How many different types of steel are there?

Steels can be categorized in many different ways, for instance based on their microstructure, mechanical properties, application and/or their carbon or alloying content. SSAB is a supplier of some steel types such as structural, pressure vessel, wear plate and tooling steels. 

20. What are the residual elements of steel?

A residual element in steel is an element not wanted, but cannot be eliminate in the steel-making process. An element can be an intentional alloying element in one grade and residual element in another grade. In general, the residual elements are coming from the input raw materials as iron ore or recycled steel scrap. The content of the unwanted residual elements in SSAB plates is always lower than the critical levels. 

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Overcoming difficulties in the metallographic preparation of high alloy tool steels

Avoiding thermal damage
As heat treatability of high alloy tool steels is a quality criterion, thermal influence during cutting has to be avoided in order to ensure a true representation of the actual microstructure. When cutting larger sections, this preparation step has to be carried out with great care.

Fig. 2: Thermal damage due to faulty cutting conditions 

Preserving carbides and inclusions
The main difficulty during grinding and polishing of high alloy tool steels is ensuring that carbides and non-metallic inclusions are retained. In cold working tool steels, the primary carbides are very large and fracture easily during grinding. In fully annealed conditions, secondary carbides are very fine and can easily be pulled out from the softer matrix.

Fig. 3: Fractured primary carbides (Mag: 200x) 

Large volume processing of high alloy tool steels
For quality control teams working within high alloy tool steel production, processing large sample volumes requires a very efficient organization of the workflow, automatic equipment and standard procedures.

Recommendations for the grinding and polishing of high alloy tool steel

When preparing high alloy tool steels for metallographic analysis, the form, size and amount of carbides must be accurately represented. In addition, non-metallic inclusions must be retained in an undeformed matrix.

• Large volumes are best processed on fully automatic grinding and polishing machines, which guarantee a fast and efficient workflow and reproducible results.
• Tool steels are hard. Therefore, fine grinding with diamond is more efficient and economical than grinding with silicon carbide foil.
• Sometimes a final oxide polish can be useful for contrasting and identifying carbides.

Table 1: Preparation method for high alloy tool steel on large automatic equipment.
DiaPro diamond suspensions can be substituted with DP-Diamond suspension P as follows: For FG with 9 μm, DP 2 with 1 μm used with DP-Blue/Green lubricant. 

Table 2: Preparation method for high alloy tool steel on table-top semi-automatic equipment.
DiaPro diamond suspensions can be substituted with DP-Diamond suspension P as follows: For FG with 9 μm, DP 1 with 3 μm, DP 2 with 1 μm used with DP-Blue/Green lubricant.

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Recommendations for the etching of high alloy tool steel

High alloy tool steel samples are usually initially examined unetched to identify inclusions and carbide size and formation. To reveal the microstructure, various concentrations of nital or picral are used.

For example, to show the carbide distribution in cold work steel, a 10% nital ensures the matrix is dark and the white primary carbides stand out. For fine globular pearlite, a brief submersion into picric acid followed by 2% nital gives a good contrast and avoids staining.

Nital etching solution:
100 ml ethanol
2-10 ml nitric acid (Caution: Do not exceed 10% of the solution as it becomes explosive!)

Picral etching solution:
100 ml ethanol
1-5 ml hydrochloric acid
1-4 g picric acid


Fig 5: Cold work tool steel etched with 10% nital, primary carbides stand out white

Fig. 6: Hot work tool steel etched with picral and nital, globular pearlite (Mag: 500x)

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