DK1445339T3

DK1445339T3 - Alloy and article with high heat resistance and high heat stability

Info

Publication number: DK1445339T3
Application number: DK04450025.4T
Authority: DK
Inventors: Devrim Dipl -Ing Dr Caliskanoglu; Kay M Eng Fisher; Reinhold Univ Prof Dipl -Ing Dr Ebner
Original assignee: Böhler Edelstahl GmbH
Priority date: 2003-02-10
Filing date: 2004-02-10
Publication date: 2016-09-26
Also published as: BRPI0400488B1; BRPI0400488A; EP1445339A1; EP1445339B1; ATA1962003A; HUE030391T2; CA2457183A1; AT411905B; PT1445339T; CA2457183C; ES2592714T3

Description

The invention relates to an alloy for the production of articles with high heat resistance and toughness.

In particular the invention relates to a hot-working steel article with good hardness, high heat resistance and high thermal stability.

In general, hot-working steels may be described as heat-treatable iron-based alloys with enhanced mechanical properties after heat treatment, in particular their good strength and hardness up to temperatures of 500°C and above, which are retained.

Corresponding to the rising demands of technical development, there is a general requirement for further improvements in the quality of hot-working materials, in particular an increase in their heat resistance with high thermal stability, combined with greater toughness.

Customary hot-working steels are iron-based alloys containing 0.3 to 0.4 weight-% carbon (C). Their hardness is increased by quench hardening through martensite forming in the structure and tempering as required. An addition of alloy elements to the iron-based material, generally in the following weight-%: Silicon (Si) up to 1.5

Chromium (Cr) 2.5 up to 5.5 Molybdenum (Mo) up to 3.0

Vanadium (V) up to 1.0 and application of specially designed heat treatment processes makes it possible to produce from this an article which has high values for desired mechanical properties at an application temperature of up to around 500°C. The application temperature may be somewhat increased by adding up to 9% by weight of tungsten (W) and up to 3% by weight of cobalt (Co) to the alloy.

Essentially, the heat resistance of such steels results from a precipitation mechanism, described by the person skilled in the art as a rise in secondary hardness, in which very fine chromium-molybdenum-tungsten-vanadium carbides are formed in the martensite grid, as for example disclosed in documents JP 07228945A and US-A-3453151. A further increase in the strength of a material, differing in substance from quench hardening, may be obtained by means of precipitation hardening. The precondition for precipitation hardening is a solubility of an alloy additive or alloy elements in the base metal which declines with temperature.

In precipitation hardening, an alloyed material is first subjected to solution heat treatment with subsequent enhanced cooling-down. By this means an alloy additive or a phase is brought wholly or partly into solution and held in supersaturated solution. Subsequent heating to a temperature below the solution heat treatment temperature effects a precipitation of the supersaturated portion of the element(s) or phase(s), resulting in a change in material properties, generally a rise in material hardness.

Iron-based materials which may be hardened by precipitation generally have an alloy content in % by weight of:

Carbon (C) up to 0.05

Manganese (Mn) up to 2.0

Chromium (Cr) up to 16.0

Molybdenum (Mo) up to 6.0

Nickel (Ni) up to 26.0

Vanadium (V) up to 0.4

Cobalt (Co) up to 10.0

Titanium (Ti) up to 3.0

Aluminium (Al) up to 0.3

Both the iron-based alloys with martensite forming during quench hardening, and those which undergo a change in their mechanical properties through precipitation of elements and phases, have the common drawback that, in the respective area of the alloy composition and/or through heat treatment technology, in each case only individual properties, such as for example hardness and strength or heat resistance, are improved, but this is linked to a decline in other property values, such as for example material toughness, thermal stability and the like.

The aim of the invention is to specify an alloy which makes it possible to improve the overall property profile of an article made from it. According to the problem of the invention, a hot-working steel article is to be created which simultaneously has high levels of hardness and toughness, high heat resistance and high thermal stability.

The objective of the invention specified above is achieved with an alloy consisting in weight-% of:

Carbon (C) 0.15 to 0.44

Silicon (Si) 0.04 to 0.3

Manganese (Mn) 0.06 to 0.4

Chromium (Cr) 1.2 to 5.0

Molybdenum (Mo) 0.8 to 6.5

Nickel (Ni) 3.4 to 9.8

Vanadium (V) 0.2 to 0.8

Cobalt (Co) 0.1 to 9.8

Aluminium (Al) 1.4 to 3.0

Copper (Cu) less than 1.3

Niobium (Nb) less than 0.35

Iron (Fe) remainder together with impurities due to production.

The benefits arising from the invention are essentially to be seen in the creation by means of measures involving alloy technology of a material in which precipitation hardening may be superimposed on quench or martensite hardening. At the same time the activities of the alloy elements in respect of the carbon and those relating to the compound or phase formation are chosen so advantageously that, even at relatively low austenitizing temperatures, hardening due to very fine secondary carbide precipitation, in particular chromium-molybdenum-vanadium-carbides, and hardening due to a precipitation of intermetallic phases, in particular of AIFe2Ni during hardening and tempering, take place simultaneously and high heat resistance combined with high toughness of the material is obtained.

According to the invention, the ability for large parts to be fully hardened may also be improved, since a suitable thermal transformation behaviour of the material is set by means of alloy technology. Likewise, tempering stability and thus the thermal stability of the heat-treated material with high hardness are significantly improved.

In an iron-based alloy according to the invention, a carbon content of at least 0.15 weight-% is provided, so that an adequate amount of carbide may be precipitated for a desired rise in secondary hardness. Carbon concentrations in excess of 0.44 weight-% may, with the carbide-forming elements provided, form disruptive primary carbides which reduce toughness, so that the carbon content should lie between 0.15 and 0.44 weight-%.

The silicon content, for an advantageous composition of a deoxidisation product, should be at least 0.04 weight-%, but otherwise no greater than 0.3 weight-%, since higher silicon values have an adverse effect on material toughness.

According to the invention, manganese is provided in the steel at a concentration of between 0.06 and 0.4 weight-%. Lower content may lead to brittleness in hot forming, and higher content may have adverse effects on hardenability of the material.

The content levels of chromium, molybdenum and vanadium are important for the desired formation of secondary hardness in the material during hardening and tempering and should be considered together. Chromium content of less than 1.2 weight-% is disadvantageous for ability of the material to be fully hardened, while values of over 5.0 weight-% reduce thermal stability since they push back the activity of the molybdenum.

With molybdenum concentrations of less than 0.8 weight-%, too little of this element is brought into solution in the course of heat treatment, leading to low secondary hardness values. More than 6.5 weight-% in the steel may produce too high a carbide content, leading possibly to a loss of material toughness and economic efficiency.

The strong carbide producer vanadium is according to the invention provided at a minimum content level of 0.2 weight-%, in order to ensure adequate and stable secondary hardening of the steel. Vanadium content in excess of 0.8 weight-% may lead to precipitation of primary carbides, especially with carbon content in the upper part of the provided concentration range, resulting in a sharp deterioration in toughness properties of the material.

The effect of niobium is certainly similar to that of vanadium, but is characterised by the formation of very stable carbides, so that the niobium content should advantageously amount to less than 0.35 weight-%.

To ensure a desired rise in secondary hardness during tempering of the martensite structure of the alloy according to the invention, the latter has, with a carbon concentration of 0.15 to 0.44 weight-%, content levels in weight-% of 1.2 to 5.0 chromium, 0.8 to 6.5 molybdenum, and 0.2 to 0.8 vanadium.

The nickel concentration of the steel and its aluminium content are to be seen, in respect of the precipitation kinetics of the phase of type AIFe2Ni, as increasing hardness with a provided heat treatment process. With nickel content of less than 3.4% weight-% and an aluminium concentration of less than 1.4 weight-%, precipitation hardening is pushed back, i.e. the additional rise in hardness as material under tempering is low.

Nickel content in excess of 9.8 weight-% shifts the γ/ a conversion to lower temperatures, which may lead to problems during soft annealing of the steel, a high machining hardness, and disturbance of the precipitation kinetics.

Aluminium content above 3.0 weight-% promotes in a disadvantageous manner a high DELTA-(5)-ferrite range in the transformation behaviour, nitride production, and reduced material toughness of the alloy.

According to the invention, therefore, the nickel content and the aluminium content of the steel in weight-% lie in the ranges 3.4 to 9.8 for nickel and 1.4 to 3.0 for aluminium.

Copper may form undesired intermetallic phases and should be contained in the steel in a low concentration of less than 1.3 weight-%.

For further improvement of the property profile of the alloy according to the invention, it may be provided for this alloy to contain one or more of the elements with the following concentrations in weight-%:

Carbon (C) 0.25 to 0.4 preferably 0.31 to 0.36

Silicon (Si) 0.1 to 0.25 preferably 0.15 to 0.19

Manganese (Mn) 0.15 to 0.3 preferably 0.2 to 0.29

Chromium (Cr) 1.9 to 2.9 preferably 2.2 to 2.8

Molybdenum (Mo) 1.2 to 2.9 preferably 2.1 to 2.9

Nickel (Ni) 5.0 to 7.6 preferably 5.6 to 7.1

Vanadium (V) 0.24 to 0.6 preferably 0.25 to 0.4

Cobalt (Co) 1.4 to 7.9 preferably 1.6 to 2.9

Aluminium (Al) 1.6 to 2.9 preferably 2.1 to 2.8

By means of these narrower content ranges of elements in the chemical composition of the steel, a further improvement in properties of articles made from the steel may be obtained.

Of particular importance for altogether high mechanical steel values, but especially also for high toughness of the material, is a limited content of impurities.

Provided in an advantageous development of the invention is an alloy containing one or more of the impurity elements with the following MAXIMUM concentrations in weight-%:

Phosphorus (P) 0.02 preferably 0.005

Sulphur (S) 0.008 preferably 0.003

Copper (Cu) 0.15 preferably 0.06

Titanium (Ti) 0.01 preferably 0.005

Niobium (Nb) 0.001 preferably 0.0005

Nitrogen (N) 0.025 preferably 0.015

Oxygen (0) 0.009 preferably 0.002

Calcium (Ca) 0.003 preferably 0.001

Magnesium (Mg) 0.003 preferably 0.001

Tin (Sn) 0.01 preferably 0.005

Tantalum (Ta) 0.001 preferably 0.0005

In order to obtain an especially pronounced precipitation hardening capability of the alloy, superimposed on the secondary hardening through carbides, it may be of advantage for the value nickel content divided by aluminium content, respectively in weight-%, to lie between 1.8 and 4.2, preferably between 2.1 and 3.9. This avoids an overhang of an element producing the precipitation .

The set problem of the invention is solved according to an improved property profile for a hot-working steel article, when a starting material made in accordance with a melt-metallurgical or powder- metallurgical process and with a chemical composition specified in advance has been brought into shape through hot working and machining. This formed article, after age-hardening heat treatment, has secondary precipitated carbides, together with intermetallic precipitations.

The overall hardness of the material is here advantageously obtained by a superimposition of the rise in secondary hardness through carbide precipitations and precipitation hardening. By this means, high material hardness values may be achieved, although the heat treatment technology is directed to the retention of high material toughness and, in contrast to a hot working steel according to the prior art, lower hardening temperatures may be used. This low austenitizing temperature may also have significant advantages in respect of minimal distortion in the hardening and tempering treatment of complex formed parts.

If however the hardening temperatures are set at a high level, this results in extremely high hardness values of the steel article, combined with otherwise customarily good material toughness.

If in the structure of the hot-working steel article, a ratio of intermetallic precipitates divided by secondary precipitated carbides, respectively in vol.-% of less than 3.0, preferably 1.0 and below, but above 0.38, is present then, with high hardness values, toughness is especially high, and thermal stability is shifted to higher temperatures by up to 50°C and more. A hot-working steel article according to the invention, which has secondary precipitated chromium-molybdenum-vanadium mixed carbides and substantially intermetallic phases of type AIFe2Ni in the structure, has an especially preferred property profile, and may be produced efficiently in conventional hardening installations with comparatively low hardening temperatures.

Pronounced thermal stability of the article may be obtained when the alloy has a ratio value of chromium + molybdenum + vanadium divided by carbon respectively in weight-% of more than 12 but less than 19.

The invention will be explained in detail, by way of example, with the aid of several test results and diagrams.

From an alloy A according to the invention, from a conventional hot-working steel B, and from a precipitation hardening steel C (maraging steel), specimens were produced, thermally hardened and tempered, and their material properties tested. The alloys have the chemical composition shown in Table 1:

Table 1

The test material first underwent measurement of thermal expansion a [10-6/K] depending on temperature, at an initial hardness of the material of 50 to 52 HRC. The values which may be taken from Table 2 show that, in comparison with a conventional hot-working steel, the alloy according to the invention has less expansion, which also indicates better dimensional stability during heat treatment.

Table 2

After hardening, in each case to around 55 HRC, of specimens from alloy A according to the invention and the hot-working steel B, the hardness pattern of the materials depending on temperature was determined. Here it is of great significance that, to obtain this hardness, alloy A according to the invention needed an austenitizing temperature of 990°C, but the conventional hot-working steel B required one of 1050°C. Depending on temperature, as evident from tables 3A and 3B, in the range between 500°C and 600°C, the hardness of the specimen A composed according to the invention rose to values around 60 HRC, whereas for the conventional hot-working steel B a maximum hardness value of 56 HRC at 500°C was determined.

Table 3A

Table 3B

Fig. 1 shows in graphical form the respective hardness pattern depending on temperature of material A according to the invention and hot-working steel alloy B according to the prior art. Starting from a similar hardness, obtained however with an advantageous lower austenitizing temperature, where applicable, alloy A according to the invention undergoes a significantly greater rise in heat resistance of the article, due to a superimposed precipitation mechanism, in which AIFe2Ni precipitates are produced in the structure in the finest form, while this heat resistance is also maintained at higher temperatures.

Based on a hardness specification according to Vickers, the testing of the softening behaviour of the material over time took place at a temperature of 650°C.

Determination of hardness of the test specimen at the testing temperature was conducted according to the rebound resilience method (Shore hardness), for which rebound values there is to date only a conversion into Vickers hardness values.

Starting out from approximately similar hardness at room temperature and specifically 50 - 52 HRC, which was obtained for the alloys A, B and C with a composition according to Table 1 through different hardening and tempering methods set out in the test Appendix results sheet 1, hardness testing over time was performed at 650°C.

Compared with a conventional hot-working steel B and a maraging steel C, the alloy A according to the invention had the highest material hardness, from an identical initial hardness of 650°C, during a period of up to 1000 minutes. After this period of time, maraging steel C had greater hardness with high thermal stability, whereas on the other hand hot-working steel A according to the invention lost around 10% of its hardness up to approximately 2000 minutes. The thermal stability of the conventional hot-working steel B was low, with the difference in hardness in comparison with alloy A according to the invention increasing steadily up to 1000 minutes.

Claims

1. Alloy for the manufacture of articles of high heat resistance and toughness consisting of, by weight:

and manufacturing-related impurities.

An alloy according to claim 1, containing one or more elements having the following concentrations in weight percent:

An alloy according to claims 1 to 3, containing one or more elements of impurities with the following maximum weight percentages:

An alloy according to any one of claims 1 to 5, wherein the value of nickel content divided by aluminum content, respectively, in weight percent, is between 1.8 and 4.2:

An alloy according to any one of claims 1 to 5, wherein the value of nickel content divided by aluminum content, respectively, in weight percent, is preferably between 2.1 and 3.9:

An alloy according to any one of claims 1 to 7, wherein, in its chemical composition, a ratio of chromium + molybdenum + vanadium is divided by carbon, respectively, by weight, of greater than 12 but less than 19:

9. A high hardness, high heat resistance and high thermal stability hot working steel article, wherein a starting material made of a metallurgical melt or powder metallurgical process, with a chemical composition characterized by the preceding claims, was molded by hot forming and machining, wherein the shaped article subsequently comprises a cure heat treatment, secondary precipitated carbides in the structure, and intermetallic precipitates.

The hot working steel article of claim 9, wherein in the structure comprises a ratio of intermetallic precipitates divided by secondary precipitated carbides, respectively, in volume percent, of less than 3.0.

A hot working steel article according to claim 9, wherein in the structure comprises a ratio of intermetallic precipitates divided by secondary precipitated carbides, respectively, in volume percent, of 1.0 or less but greater than 0.38.

A hot working steel article according to claims 9 to 11 comprising chromium-molybdenum-vanadium-mixed carbides secondarily precipitated in the structure and intermetallic phases of the AIFe2Ni type in the structure.