WO2020050727A1

WO2020050727A1 - Steel alloy

Info

Publication number: WO2020050727A1
Application number: PCT/NZ2019/050117
Authority: WO
Inventors: Abrie VAN AARDE
Original assignee: Milson Foundry Nz Limited
Priority date: 2018-09-07
Filing date: 2019-09-06
Publication date: 2020-03-12
Also published as: US20210189514A1

Abstract

A cast steel alloy comprising carbon (0.8 to 2.0%), chromium (4 to 15%), silicon (0.68 to 2.0%), manganese (0.6 to 1.2%) and nickel (1.5 to 4%) that exhibits exceptional hardness and tensile strength and is useful for a wide range of high wear resistance applications including in the mining, excavation and agriculture industries.

Description

STEEL ALLOY

TECHNICAL FIELD

The invention relates generally to steel alloys. In particular, the invention relates to a cast steel alloy that exhibits exceptional hardness and tensile strength and is useful for a wide range of high wear resistance applications.

BACKGROUND OF THE INVENTION

Common alloys in the fields of wear resistant materials are white irons, alloy steels and stainless steels. White irons are formed when alloying elements, such as chromium, is added to cast iron in an alloying process. The presence of silicon forces some carbon out of solution, but carbon remaining in solution forms an iron carbide precipitate (cementite) exhibiting white fractured surfaces. Alloy steels are alloys of iron and carbon, and usually other elements. Carbon in a steel alloy is typically present in the amount of up to 2.14% by weight of the alloy. Stainless steels are steel alloys that contain at least 11% by weight chromium. They often also contain nickel for corrosion resistance.

Each of the above materials has its own limitations. Irons have a low tensile strength and low wear resistance, with the exception of white irons and austempered ductile iron (ADI). White irons, which are the most wear resistant irons, have induced brittleness, low impact resistance and very low elongation. In the field of steels, it is possible to achieve a tensile strength exceeding 600 MPa, but there is limited scope to create wear resistance utilising the wear resistant characteristic of ferro chrome (an alloy of chromium and iron containing 50- 70% chromium). Stainless steels are not used as a manufacturing material because of the high cost of production and because of their primary characteristics of corrosion resistance and heat resistance.

In the applications of ground engaging, mining, and aggregate parts, there is a need for high wear resistance, impact resistance, and plastic deformation at a reasonable cost. One challenge all materials face is having a stable metallic structure which can perform in all aspects of the above mentioned characteristics. In metallurgy, there is always a direct trade- off between toughness and hardness. A ductile material will have a low hardness and a wear resistant material will have a high hardness. Specifically, iron alloys fall into two categories; steels (with a maximum carbon content of 2%) and irons (with a minimum carbon content of 2.01%). With the exceptions of tool steel and Hadfield manganese steel, steels have a much lower wear resistance compared to white irons. The raw material for steel manufacturing is generally more costly than the raw material for iron manufacturing due to the low-carbon content of steels and the cost of accompanying alloys such as nickel, molybdenum, cobalt and tungsten. All steels are also required to undergo a heat treatment process after casting which further adds to the cost. Hadfield manganese steel is an exception because, after quenching, this steel will work-harden under impact in a wear resistance application to increase the life of wearing parts while utilising the low-costs elements of Manganese and Chromium. Hadfield manganese steels are also lower-cost steels in comparison to stainless steels, but they have a higher manufacturing cost than carbon steels due to multiple heat treatment processes and alloying costs. Irons have a much lower tensile strength and wear resistance (with the exception of white irons and ADI). Low tensile strength (less than 250 MPa) limits the use of the alloy to applications where the part is subjected to forces lower than the tensile range. White irons, which are the most wear resistant irons, have induced brittleness, low impact resistance and very low elongation.

The trade-off between toughness and hardness provides a challenge to the development of an alloy able to deliver high wear resistance but also have at least limited deformation ability prior to failure. The applicant has now found a steel alloy that is effective in the casting of the ground engaging parts and the physical effects of the conditions (wear, corrosion, impact and strain) encountered by the alloy while in commission.

It is therefore an object of the invention to provide a cast steel alloy, or at least to provide a useful alternative to existing steel alloys.

SUMMARY OF INVENTION

In a first aspect of the invention there is provided a cast steel alloy comprising iron and the following :

a) carbon in the amount of 0.8 to 2.0% by weight of the alloy;

b) chromium in the amount of 4 to 15% by weight of the alloy;

c) silicon in the amount of 0.68 to 2.0% by weight of the alloy;

d) manganese in the amount of 0.6 to 1.2% by weight of the alloy; and

e) nickel in the amount of 1.5 to 4% by weight of the alloy.

In some embodiments of the invention, the carbon is present in the amount of 1.2 to 1.4% by weight of the alloy.

In some embodiments of the invention, the chromium is present in the amount of 10 to 12% by weight of the alloy.

In some embodiments of the invention, the silicon is present in the amount of 1.0 to 1.2% by weight of the alloy.

In some embodiments of the invention, the manganese is present in the amount of 0.95 to 1.05% by weight of the alloy.

In some embodiments of the invention, the nickel is present in the amount of 1.9 to 2.2% by weight of the alloy.

In certain embodiments, the alloy further comprises any one or more of: a) aluminium in the range of 0 to 0.15% by weight of the alloy;

b) molybdenum in the range of 0 to 2.4% by weight of the alloy; and

c) tungsten in the range of 0 to 0.5% by weight of the alloy;

The alloy of the invention may further comprise any one or more of vanadium, copper, cobalt, niobium, phosphorus, and sulfur.

In one embodiment of the invention, the alloy comprises:

a) carbon in the amount of 1.42% by weight of the alloy;

b) chromium in the amount of 11.67% by weight of the alloy;

c) silicon in the amount of 1.28% by weight of the alloy;

d) manganese in the amount of 0.98% by weight of the alloy;

e) nickel in the amount of 2.15% by weight of the alloy; and

f) molybdenum in the amount of 0.50% by weight of the alloy;

with the remainder of the alloy comprising one or more of vanadium, copper, cobalt, niobium, phosphorus, and sulfur.

In some embodiments of the invention, the alloy has a Brinell hardness of 340 BHN to 420 BHN, and/or a Rockwell hardness of 35 to 45 HRC, and/or a tensile strength of 400 to 500 MPa.

In a second aspect of the invention there is provided a process for preparing a cast alloy of the invention comprising the steps:

i) preparing molten iron;

ii) adding chromium, silicon, manganese and nickel and mixing to form an homogenous melt;

iii) deoxidising the melt; and

iv) allowing the melt to cool and form the alloy.

In some embodiments of the invention, the process further comprises heat treatment of the alloy.

In some embodiments the alloy is normalised by heating to 900 °C for approximately 3 hours then allowed to cool to ambient temperature.

In other embodiments the alloy is quenched by heating to 900 °C for approximately 3 hours then submersed in air, oil or water.

In some embodiments of the invention, the alloy that has undergone heat treatment has a Brinell hardness of 550 to 650 BHN, and/or a Rockwell hardness of 50 to 65 HRC, and/or a tensile strength of 700 to 800 MPa.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is an image at 100 pm scale of Sample 5 treated with Vilella's Etchant.

Figure 2 is an image at 100 pm scale of Sample 3 treated with Vilella's Etchant. Figure 3 is an image at 300 pm scale of Sample 4 treated with Vilella's Etchant.

Figure 4 is an image at 25 pm scale of Sample 6 treated with Vilella's Etchant.

Figure 5 is an image at 100 pm scale of Sample 7 treated with Vilella's Etchant.

Figure 6 is an image at 100 pm scale of Sample 8 treated with Vilella's Etchant.

DETAILED DESCRIPTION

Definitions

The term "steel" means an alloy of iron containing carbon in the amount 0.002 to 2.00% by weight.

The term "carbon steel" means an alloy of iron and carbon in the absence of other alloyed elements.

The term "steel alloy" means steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium.

The term "cast steel alloy" means a steel alloy that has been prepared by casting, i.e. by pouring in a molten state into a solid vessel and then allowed to solidify.

The term "tensile strength" means the resistance of a material to breaking under tension.

The term "hardness" means the resistance to localised plastic deformation induced by mechanical indentation or abrasion and is typically measured on the Rockwell scale (HRA, HRB, HRC, etc.) or the Brinell scale.

The term "austenitic steel" means is a form of steel having a face-centred cubic crystal structure usually stabilised with nickel, manganese and nitrogen, and which is not hardenable by heat treatment and is non-magnetic.

The term "martensitic steel" means a form of steel having a body-centred tetragonal crystal structure which is formed by quenching austenitic iron at a high rate leaving the steel supersaturated with carbon. Martensitic steel has a high degree of hardness.

The term "anneal" means heating an alloy to its critical temperature to cause a crystalline phase change from ferrite to austenite then cooling slowly in an insulating environment allowing formation of cementite.

The term "normalise" means heating an alloy to its critical temperature to cause a crystalline phase change from ferrite to austenite then cooling in open air to enable formation of pearlite, but avoid formation of cementite. The alloy of the invention

The invention provides a cast steel alloy comprising iron and the elements carbon, chromium, silicon, manganese and nickel in amounts that result in certain beneficial properties of the alloy such as high wear resistance, tensile strength, and hardness. The amounts of the alloy elements are:

a) carbon in the range 0.8 to 2.0% by weight of the alloy;

b) chromium in the range 4 to 15% by weight of the alloy;

c) silicon in the range 0.68 to 2.0% by weight of the alloy;

d) manganese in the range 0.6 to 1.2% by weight of the alloy; and

e) nickel in the range 1.5 to 4% by weight of the alloy.

Importantly, the alloy of the invention is a cast alloy. The alloy possesses beneficial properties without the need for the alloy to undergo further processing steps, although it will be appreciated that the alloy of the invention maybe further processed.

The alloy can be used to construct a material with the desired integrities of a stable metallic structure without further processing, such as heat treatment. A solidification curve for the alloy shows that allow percentage of residual stress in the casting which in turn complements the tensile strength of the alloy. The chemical composition of the alloy promotes carbide formation leading to an austenitic/martensitic mixed structure. The alloy was found to have a uniform equiaxed structure (crystals having axes of the same length) throughout the casting.

The amounts of carbon, chromium, silicon, manganese and nickel in the alloy may vary within the abovementioned ranges. For example, the carbon may be present in the amount of 1.2 to 1.4% by weight of the alloy. The chromium may be present in the amount of 10 to 12% by weight of the alloy. The silicon may be present in the amount of 1.0 to 1.2% by weight of the alloy. The manganese may be present in the amount of 0.95 to 1.05% by weight of the alloy. The nickel is present in the amount of 1.9 to 2.2% by weight of the alloy.

One example of an alloy of the invention comprises:

a) carbon in the amount of 1.42% by weight of the alloy;

b) chromium in the amount of 11.67% by weight of the alloy;

c) silicon in the amount of 1.28% by weight of the alloy;

d) manganese in the amount of 0.98% by weight of the alloy;

e) nickel in the amount of 2.15% by weight of the alloy; and

f) molybdenum in the amount of 0.50% by weight of the alloy.

The remainder of the alloy composition will depend primarily on the raw material used in the process for preparing the alloy, and the process conditions used, and will typically comprising one or more of vanadium, copper, cobalt, niobium, phosphorus, and sulfur in trace amounts. Additional examples of alloys of the invention are set out in the Examples.

Hardness and tensile strength are important characteristics of the alloy of the invention. The alloy may have an "as cast" hardness of 340 to 420 BHN, meaning that the alloy has a hardness in this range without any heat treatment after casting. On the Rockwell scale, the alloy may have a hardness of 35 to 45 HRC. The alloy may have a tensile strength of 400 to 500 MPa.

If the alloy is taken through a heat treatment process after casting, a Brinell hardness of 550 to 650 BHN, and/or a Rockwell hardness of 50 to 65 HRC, and/or a tensile strength in excess of 700 can be expected.

Surprisingly, certain alloys of the invention exhibit high hardness (e.g. 600 BHN) without having been subjected to heat treatment. See Example 5 below.

The most common heat treatment processes are annealing, quenching and tempering. Annealing involves heating the steel to a temperature sufficiently high to relieve stresses in the steel. The temperature required will depend on the alloy constituents. Quenching involves heating the steel to an austenite phase and then quenching with water or oil. Rapid cooling causes the formation of a hard but brittle structure. Tempering is a specialised type of annealing used to reduce brittleness in the structure. The alloy is heated to a temperature below its critical point for a period of time and then allowing it to cool in air. The heating temperature and the specific composition of the alloy determines the degree of hardness reduced.

The alloy of the invention has the ability to work-harden under impact. Work-hardening is a process where the hardness, yield strength, and tensile strength of an alloy is increased by subjecting the alloy to machining or impact of some type. The incorporation of manganese in a ratio of approximately 1 : 1 with carbon in the alloy of the invention, similar to Hadfield Manganese Steel, assists work-hardening.

Advantageously, the alloy of the invention can be classified as a stainless steel and a carbon steel. The alloy is a stainless steel due to its material due to its chromium content in the range 4 to 15% (e.g. 10%), and the alloy is a carbon steel due to its carbon content in the range 0.8 to 2.0% (i.e. greater than 0.6%).

The alloy of the invention has been found to have high wear resistance, good tensile strength, high hardness, good welding properties and good corrosion resistance.

Manufacturing process

The general steps of the manufacturing process are as follows:

1. Standard melting procedure for steel.

2. All alloy elements including chromium, nickel and molybdenum are added 20 minutes prior to completion of melting process to minimise melting losses. 3. Ensure melt is homogeneous. The melting temperature can be determined by referring to the carbon iron phase diagram for the composition, carbon content and eutectoid.

4. Deoxidise the melt by adding a stabilising deoxidant such as aluminium, calcium carbide, Zircomet or tungsten.

Application areas

The steel alloy of the invention may be used in a wide range of applications, including but not limited to the following :

• Ground engaging parts (e.g., ripper teeth)

• Mining and excavation (e.g., excavation teeth, rock-processing machinery, crushers, power shovels)

• Agriculture (e.g., plough shares, chisels, ripper points)

• Processing equipment subjected to wear (e.g., hammer mills, auger liners, conveyor liners)

• General engineering (e.g., wear resistant bushes, slide plates)

• Crushing mills (e.g., for aggregate, fertiliser, soil medium)

It is intended that reference to a range of numbers disclosed herein (for example 1 to 10) also incorporates reference to all related numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

Any reference to prior art documents in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field.

As used in this specification, the words "comprises", "comprising", and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean "including, but not limited to".

Although the invention has been described by way of example, it should be appreciated that variations and modifications may be made without departing from the scope of the invention as defined in the claims. Furthermore, where known equivalents exist to specific features, such equivalents are incorporated as if specifically referred in this specification. EXAMPLES

Example 1: Alloy manufacturing process

Mild steel (110 kg) was added into a melting furnace together with 304 stainless steel (50 kg), high carbon ferro chrome (30 kg), ferro molybdenum (3 kg), high carbon manganese flakes (0.75 kg), ferro silicon (2 kg), and recarboriser (1.2 kg). The mixture was heated until molten and observed to be homogeneous. The homogeneous molten mix poured was then poured into a compacted sand mould and allowed to cool and solidify typically over a period of up to 30 minutes, although for several hours in some instances. The alloy sample was then removed from the mould for analysis of its elemental composition and hardness determination.

Example 2: Sample 1

An alloy of sample 1 was prepared according to Example 1 and was found to have the composition shown in Table 1. The hardness of the sample was determined to be 650 BHN.

Table 1: OES analysis

Hardness and tensile strength of samples of an alloy of the invention were determined using an Avery test machine, a measuring projector, a digital micrometer, and digital callipers. Two samples were tested, an "as cast" (AC) sample and a normalised (NOR) sample. The results are shown in Table 2.

Table 2: Hardness and strength analysis

nq

Example 3: Sample 2

An alloy of sample 2 was prepared according to Example 1. This alloy was prepared using less ferro chrome to give an alloy having reduced chromium and carbon content, and was found to have a mid-range hardness of 341 HBN.

Table 3: OES analysis

Brinell hardness: 341 HBN Example 4: Samples 3-5

An alloy of sample 3 was prepared according to Example 1 where the casting was left to cool in the mould so that the temperature of the alloy followed a normal cooling curve. This resulted in an alloy with a hardness of 340 HBN. Sample 4 was prepared in the same manner except that 40 minutes after pouring, the alloy was removed from the sand mould and cooled in ambient air. A harder alloy (495 HBN) was produced under this reduced cooling curve.

Sample 5 was prepared in the same manner as for sample 3. Sample 5 is a standard high chromium alloy and was used for comparison purposes. The compositions of each sample are shown in Table 4 and the hardness of each sample in Table 5.

Table 4: OES analysis

Table 5: Hardness

Figure 1 shows martensitic, carbide and dendritic formations in the Sample 5 alloy which are indicative of a standard chrome iron having a high level of wear resistance. Figure 2 shows the well-formed, aligned and dense structure of the Sample 3 alloy consistent with a lower hardness.

Figure 3 shows the Sample 4 alloy. A comparison of Figures 2 and 3 show the transformation from "as cast" structure of Sample 3 to a heat treated structure of Sample 4. Heat treatment increases the hardness of the alloy.

Example 5: Sample 6

An alloy of Sample 6 was prepared according to Example 1 using reduced ferro chrome, carbon, manganese and silicon additives. The alloy produced has the elemental composition as shown in Table 6. The alloy has a low carbon content (1.08%) and a low Chromium content (9.85%) relative to other samples made and tested. The alloy was determined to have a surprisingly high hardness of 600 HBN even though the alloy was not subjected to heat treatment. The alloy has a more uniform and aligned microstructure (as shown in Figure 4) resulting in high hardness and therefore increased wear resistance. Importantly, this alloy represents a low cost, easy to manufacture alloy having high hardness and wear resistance properties.

Table 6: OES analysis

Brinell hardness: 600 HBN

Example 6: Samples 7 and 8

Alloy Sample 7 was prepared according to Example 1 by adding additional ferro chrome (7% more) to the molten mix. The alloy produced has the elemental composition as shown in Table 7. The alloy was determined to have a hardness of 341 HBN.

Sample 8 was prepared according to Example 1, but includes a heat treatment process. The molten mix was heated to 1050 °C by increasing the temperature 50-80 °C per hour and the temperature kept at 1050 °C for 8 hours before rapid cooling by air-quenching.

Table 7: OES analysis

Table 8: Hardness

Figure 5 shows Sample 7 having a uniformed and aligned microstructure consistent with a hardness of 341 HBN. Figure 6 shows Sample 8 having dendritic structures consistent with a high degree of hardness and wear resistance.

Claims

1. A cast steel alloy comprising iron and the following :

a) carbon in the amount of 0.8 to 2.0% by weight of the alloy;

b) chromium in the amount of 4 to 15% by weight of the alloy;

c) silicon in the amount of 0.68 to 2.0% by weight of the alloy;

d) manganese in the amount of 0.6 to 1.2% by weight of the alloy; and

e) nickel in the amount of 1.5 to 4% by weight of the alloy.

2. An alloy as claimed in claim 1, the carbon is present in the amount of 1.2 to 1.4% by weight of the alloy.

3. An alloy as claimed in claim 1, the chromium is present in the amount of 10 to 12% by weight of the alloy.

4. An alloy as claimed in claim 1, the silicon is present in the amount of 1.0 to 1.2% by weight of the alloy.

5. An alloy as claimed in claim 1, the manganese is present in the amount of 0.95 to 1.05% by weight of the alloy.

6. An alloy as claimed in claim 1, the nickel is present in the amount of 1.9 to 2.2% by weight of the alloy.

7. An alloy as claimed in any one of claims 1 to 6, further comprising any one or more of: a) aluminium in the range of 0 to 0.15% by weight of the alloy;

b) molybdenum in the range of 0 to 2.4% by weight of the alloy; and

c) tungsten in the range of 0 to 0.5% by weight of the alloy.

8. An alloy as claimed in any one of claims 1 to 7, further comprising any one or more of vanadium, copper, cobalt, niobium, phosphorus, and sulfur.

An alloy as claimed in any one of claims 1 to 8, comprising :

a) carbon in the amount of 1.42% by weight of the alloy;

b) chromium in the amount of 11.67% by weight of the alloy;

c) silicon in the amount of 1.28% by weight of the alloy;

d) manganese in the amount of 0.98% by weight of the alloy;

e) nickel in the amount of 2.15% by weight of the alloy; and f) molybdenum in the amount of 0.50% by weight of the alloy;

10. An alloy as claimed in any one of claims 1 to 9, which has a Brinell hardness of 340 BHN to 420 BHN.

11. An alloy as claimed in any one of claims 1 to 9, which has a Rockwell hardness of 35 to 45 HRC.

12. An alloy as claimed in any one of claims 1 to 9, which has a tensile strength of 400 to 500 MPa.

13. A process for preparing a cast alloy as claimed in claim 1 comprising the steps:

i) preparing molten iron;

iii) deoxidising the melt; and

iv) allowing the melt to cool and form the alloy.

14. A process as claimed in claim 13, further comprising heat treatment of the alloy.

15. A process as claimed in claim 14, wherein the alloy is normalised by heating to 900 °C for approximately 3 hours then allowed to cool to ambient temperature.

16. A process as claimed in claim 14, wherein the alloy is quenched by heating to 900 °C for approximately 3 hours then submersed in air, oil or water.

17. An alloy prepared by a process as claimed in claim 14, which has a Brinell hardness of 550 to 650 BHN.

18. An alloy prepared by a process as claimed in claim 14, which has a Rockwell hardness of 50 to 65 HRC.

19. An alloy prepared by a process as claimed in claim 14, which has a tensile strength of 700 to 800 MPa.