GB2167438A - A method of heat treating high chromium cast ferrous-based alloys and a wearing element formed of a high chromium cast ferrous based alloy - Google Patents

Abstract

A method of heat treating a component formed of an alloy consisting of 11-28 wt% chromium, 1-3.6 wt% carbon and at least 0.2 wt% molybdenum and/or at least 0.6 wt% tungsten, the remainder (apart from any incidental ingredients and impurities) being iron, comprising the steps of:- a) holding said component at a temperature between the solidus temperature of the alloy and 1050 DEG C for a period of time not exceeding 3 days in a non-oxidising atmosphere or vacuum so as to produce a partially spheroidised hard carbide phase in an austenitic matrix throughout the component, and b) quenching the component at a mean rate of from 40 DEG C/min to 3 DEG C/min in the critical temperature range in order to retain the austenitic matrix.

Description

1 GB 2 167 438 A 1

SPECIFICATION

A method of heat treating high chromium cast ferrous-based alloysand awearing elementformed of a high chromium castferrous based alloy The present invention relates to a method of producing tough, wear resistant components from a high chromium cast ferrous-based alloy and to a wearing elementwhich is formed of a high chromium ferrous-based alloy and which is for a breaking, grinding or crushing machine.

There exists a number of material handling proces ses which require the use of a tough, wear resistant material. Such applications include swing hammers used forthe crushing of building or quarried material ie. cement, clinker and limestone, wherein the hammer material must be capable of withstanding the wear conditions imposed bythefeed material and atthe same time be sufficientlytough to prevent 85 fracture due to the severe impacts encountered duringservice.

However, traditionally alloyed and processed high chromium white cast irons do not possess sufficient toughness to withstand high impact conditions.

Anumberof attempts have been madeto produce a material combiningthe required degreeof tough ness and wear resistance. These materials have frequently been in the form of composites, in which an initiallytough but malleable core is combinedwith a hard, wear resistant work face. Typical examples of thistechnique include the hard facing of steels and the fabrication of wear resistant cast iron onto tough steel shanks.

A methodfor improving thetoughness of high chromium cast irons has been previously proposed (Suieyman Butent Biner "The Effects of Metallurgical Variables on the Mechanical Properties of HighChromium Cast Irons" PhD thesis, The University of kston in Birmingham Sept. 1981) in which the high-chromium cast iron includes an addition of molybdenum ortungsten as a catalystfor enabling the production of spheroidised carbides upon heat treatment. However, whilstsuch a proposed process is capable of producing good laboratory samples, it was foundto be inappropriate forthe commercial production of industrial cast components such as the wearing elements of breaking, crushing and grinding machines (e.g. swing hammers).

In orderto obtain the microstructure of a partially spheroidised hard carbidephase supported by an austenitic matrix in industrial castcomponents, we havefound that, afterthe chemical addition of molybdenum ortungsten and holding at an elevated temperature, it is necessary to control the cooling rate of the component so thatthe austenitic matrix is retained in astable statethroughout the section of the component.

Therefore, in accordance with the present inven- tion there is provided a method of heat treating a componentformed of a high-chromium castferrousbased alloyconsisting of 11 - 28wt% chromium, 1 3.6wt% carbon and at leastO.2wt% (preferably 0.2 to about4wt%) molybdenum and/orat least 0.6wt% (preferably 0.6 to about 6 wt%) tungsten the remain- der (apart from any incidental ingredients and impurities) being iron, said method comprising the steps of:a) holding said component at a temperature between the solidus temperature of the alloy and 10500C fora period of time not exceeding about3 days in a non-oxidising atmosphere or vacuum so as to produce a partially spheroidised hard carbide phase in an austenitic matrix th roughoutthe corn po- nent, and b) quenching the component at a mean rate of from 400C/min toWC/min in the critical temperature range (as defined hereinafter) in order to retain the austenitic matrix.

Also according to the present invention, there is provided a wearing elennent of a breaking, crushing or grinding machine, said wearing element being (a) formed of a high chromium, ferrous-based alloy consisting of 11-28wt% chromium, 1-3.6wt% carbon and at leastO.2wt% (preferably 0.2to about4wt%) molybdenum and/orat leastO.6wt% (preferablyO. 6 to about 6 wt%) tungsten, the remainder (apart from any incidental ingredientsand impurities) being iron, and (b) having a stable austenitic matrix in which is dispersed a partially spheroidised hard carbide phase.

Before heattreatment, carbon and chromium form a hard sharp, angular carbide phase (see Fig. 1) and the molybdenum and/ortungsten servesto increase the rate of change in the morphology of the carbide phase during heattreatment from sharp angularity (as shown in- Fig 1) to a partly spheroidised morphology (see Fig 2). Approximately 2.5 times more tungsten than molybdenum is required to achieve similar toughness results.

As incidental ingredients, the following maybe present (inwt%):- silicon manganese nickel copper sulphur phosphorous 0-1.5 0-1.5 0-1.5 0-1.2 0-0.05 0-0.05 Of the incidental ingredients, Si, Mn, NI, and Cu all form a solid solution withinthe matrix. Hence the diffusion controlled transformation of the austenitic matrix is retarded.The remaining two elements, S and P have a deleterious effect on the toughness and are therefore maintained at low levels. They are not deliberate alloying additions.

Preferably, the chromium content of the allay is 1,4--20wt%.

It is also preferred that the carbon content of the alloy is 15- 3wt%, more preferably 2 - 3 wt%.

Some of the carbon and chromium form a solid solution in austenite and thereby influence the matrix transformation characteristics. In addition, thesetwo elements are of paramount importance in determining the volumefraction of hard carbides, M K),within the microstructure of the material.

The volume fraction of hard carbides is given bythe following relationship (Ref. F. Maratray, 1971, A.F.S.

Transactions):- 2 GB 2 167 438 A 2 %K = 12.33 (%C) + 0.55 M Cr) - 15.2 Preferablythe molybdenum contentwhen present is 1 -4wt% and thetungsten content, when present, is 2 - 5.5 wt%.

When heating the component from ambient tem peratu re to the tem peratu re of heat treatment, the heating rate shou ld be sufficiently slow to avoid cracking of the castings.

Typically, the component is heated to a treatment temperature of 11 80'C and maintained atthis temperature for up to 72 hours, preferably 4to 24 hours, most preferably 4to 10 hours. However, for reasons of cost it is desirable to minimise the heat treatment time. Furthermore, heat treatment for longer than 72 hours produces no significant improvement in properties over an extended heattreatment on an alloy containing no molybdenum ortungsten as catalyst.

Quenching of the component must be sufficiently fastto retain an austenitic matrix. The quenching rate in the critical temperature range is the important factor. The limits of the critical temperature range are defined by the treatment temperature employed in C si S p Mn 2.35 0.36 0.030 0.037 0.32 The heattreatment is carried out in an Ipsen 924 vacuum furnaceto the following schedule: heatfrom ambientto 7500C at 1500C/hour heatfrom 750'Cto 1180'C at250'Clhour held at 1 180'Cfor8 hours quench through the critical temperature range atan average rate of 3.03'Clminute using a fan assited nitrogen backfill.

Thetemperature ofthefurnace load (20castings arranged as a single layer) is measured using a load thermocouple positioned as near as possibleto the centre of the load (Le the slowest part of the loadto heat up and cool down). In this example, each casting has a pivot hole therein and the load thermocouple is placed in that pivot hole which is the nearest to the centre of the load.

The material exhibits the following typical mechanical properties:

Hardness: 420 Hv30 Fracture Toughness: using a short bar specimen tested in accordance with S.A.E.

step a) above and the upper critical temperature (A3). The quenching rate is the mean cooling rate of the furnace load, for example as measured by a thermocouple inserted into the furnace load.

The heat treatment used in the present invention may also be carried out under pressure e.g. by hot isostatic pressing, typically at 105 MPa.

It is preferred thatthe quenching rate in the critical temperature range is 37'Clmin to 3'Clmin. The non-oxidising atmosphere mayfor example be a protective atmosphere such as nitrogen. In the case wherethe holding attemperature is effected in a vacuum, it is preferred to effect quenching by back filling with an inert gas such as nitrogen.

The quenching rate in the critical temperature range is typically affected by the factors such as size of furnace load, type of furnace etc.

The present invention will now be further described in the following Examples. EXAMPLE 1 Castings having a section size of between 40 m m and 100 mm are produced with thefoliowing chemical composition (wt%):- Ni Cr Cu Mo Fe 0.19 14.05 0.16 1.92 Balance TARLE 1 ALLOYING ELEMENTS (WT EXAMPLE C si S p Mn Ni Cr Cu M W 2 2.79 0.66 0.017 0.028 0.29 0.12 16.0 0.14 0.69 - 3 2.60 0.68 0.018 0.030 0.28 0.12 15.7 0.15 2.23 - 4 2.60 0.72 0.020 0.031 0.27 0.12 15.2 0.15 3.98 - 2.79 0.62 0.016 0.028 0.31 0.12 15.5 0.15 - 6 2.75 0.63 0.017 0.028 0.31 0.12 15.2 0.15 - 7 2.70 0.65 0.017 0.034 0.31 0.12 14.8 0.16 - In Table 1 above, the balance of the ingredients is iron.

Table 2 below identifies the hardness and toughness values exhibited by the above materials after the specified times at 11SOC.

Table 2

Time at EXAMPLE Temperature (hours) 3 4 4 a 24 4 a 24 4 a 24 507 515 515 Hardness Fracture (EV3O) Toughness (Malc312) "I 25% 25.0 26.3 505 478 441 474 465 438 28.8 30.1 27.8 27.4 26.3 24.0 ARP 1704 5 a 516 24.3 KicsB 44.0 MN/m 312 24 510 23.1 6 a 506 27.3 24 475 25.3 KicsB isthe plane strain fracture toughness as determined using a short bar specimen. EXAMPLES2-7 Varioussamples of high chromium castiron having alloying ingredients as set out in Table 1 below are heatedto 11 80'C at an average rate of 290'Clhour and quenched at an average rate of 36'Clminute throug h the critical temperature range.

1.15 3.09 5.43 7 a 24 448 440 30.4 27.7 Comparative Example 1 A high chromium cast iron having the following chemical composition in wt%:- 3 GB 2 167 438 A 3 c si S p Mn 2.59 0.53 0.042 0.034 0.59 is heated in vacuum from ambientto 7500C at 150'C/hourfrom 750OCto 11 80'C at 250OC/hour, held at 1 180'C for8 hours and then quenched at an average rate of 2.50C/minute through the critical temperature range.

Thisfailsto producethe required matrix micros tructure, see Fig. 3, and combination of mechanical properties. Due to the slow cooling rate in the critical temperature range, the matrix is destabilised bythe c si S p 0.82 0.44 0.044 0.031 is heated to 11 500C for 8 hours results in the segregation of carbides into an embrittling grain boundary film. This is due to the low carbon content. 25 wt%:

Comparative Example 3 c si S p Mn 2.79 0.65 0.017 0.028 0.28 does not exh i bit satisfacto ry toug h ess o r ha rd ness pro perties after bei n g su bjected to the 8 hou r treatment described in Example 2 - 7 (Table 2).

This material has a microstructure in which the absence of Mo orW has resulted in the absence of spheroidisation and consequently this material is not satisfactory.

Hardness typically: HV30 - 596 Fracture Toughness typically: KIc- 18.6 MN/m 312 Comparative Example 4 The materials of Examples 3 and 6 are heattreated as stated butfor an extended period of over about 72 hours. Afterthis extended treatment, it isfound that the beneficial catalystic effect of the molybdenum or tungsten addition is lost.

A material within the preferred ranges regarding other components but with a carbon content in 85 excess of 3.6% by weight contains a significant volume fraction of large primary chromium carbides which promote an embrittling effectwithin the material.

If the chromium content is less than 11 wt%, a low volumefraction of [yard chromium carbides is formed. Ifthe chromium contentfallsto about8wt% (or less)then the chemical composition of the carbide phase changes and a less hard carbide isformed.

At higher chromium levelsthan 28wt%,there is a profound tendencyto form a soft primaryferrite phase.

Claims (15)

1. A method of heat treating a component formed of a high-chromium cast ferrous-based alloy consist ingof 11 -28wt% chromium, 1 -3.6wt% carbon and at least 0.2 wt% molybdenum andlor at least 0.6 wt% tungsten, the remainder (apart from any incidental ingredients and impurities) being iron, said method comprising the steps of:

a) holding said component at a temperature be tween the solidus temperature of the alloy and 1050'C fora period of time not exceeding 3 days in a non-oxidising atmosphere or vacuum so as to produce a partially spheroidised hard carbide phase in an austerritic matrix throughout the component, Ni Cr Cu M0 Fe 0.18 15.9 0.17 2.22 balance formation of secondary carbides. This results in partial transformation of the matrix to martensite, thus rendering the material unsuitable for use under high impact conditions.

Hardness: typically HV30 - 690 Fracture Toughness, typically: KicsB - 45.0 MN1m312 Comparative Example 2.

A h ig h ch rom i u m ferrous-a 1 loy conta i n i n g the following alloying ingredients in wt%.- Mn Ni Cr Fe 0.56 0.18 16.3 balance Atungsten and molybdenum-free, high chromium cast iron containing the following ingredients in Ni Cr Cu Fe 0.12 16.2 0.13 balance and b) quenching the component at a mean rate of from 405C/min to WC/min in the critical temperature range in orderto retain the austenitic matrix.

2. A method as claimed in claim 1 wherein the alloy contains as incidental ingredients, the following (in wt%):- silicon manganese nickel copper sulphur phosphorous 0-1.5 0-1.5 0-1.5 0-1.2 0-0.05 0-0.05

3. A method as claimed in claim 1 or2whereinthe molybdenum content of the alloy isO.2-4wt%.

4. A method as claimed in claim 3 wherein the molybdenum content of the alloy is 1 - 4 wt%.

5. A method as claimed in any preceding claim wherein the tungsten content of the alloy isO.6 - 6 Wt%.

6. A method as claimed in c[aim 5 wherein the tungsten content of the alloy is 2 - 5.5 wt%.

7. A method as claimed in any preceding claim wherein the chromium content of the a lioy is 14 -20 WM

8. A method as claimed in any preceding claim wherein the carbon content of the alloy is 15-3wt%.

9. A method as claimed in claim 8 wherein the carbon content of the alloy is 2 -3wt%.

10. A method asclairned in any preceding claim wherein the component is heated to a treatment temperature of about 1180'C.

11. A method as claimed in any preceding claim, wherein the component is maintained at the treat- ment temperature for up to 24 hours.

12. A method as claimed in claim 11 wherein the component is maintained at the treatment temperaturefor4to 24 hou[UNASSIGNED CODE 3DIrs.

13. A method as claimed in claim 12 wherein the component is maintained at the treatment temperaturefor4W 10 hours.

4 GB 2 167 438 A 4

14. A method as claimed in any preceding claim wherein the quenching rate is 37'Clmin to 30C/min.

15. A wearing element of a breaking, crushing or grinding machine, said wearing element being (a) formed of a high chromium, ferrous-based alloy consisting of 11 -28wt% chromium, 1 -3.6wt% carbon and atleastO.2wt% molybdenum andlorat least 0. 6 wt% tungsten, the remainder (apart from any incidental ingredients and impurities) being iron, and (b) having astable austenitic matrix in which is dispersed a partially spheroidised hard carbide phase.

Printed in the United Kingdom for Her Majesty's Stationery Office. 8818935, 5186 18996. Published at the Patent Office, 25 Southampton Buildings, London WC2A lAY, from which copies may be obtained.