GB1562788A - Production of metal articles from tool steel or alloy steel powder - Google Patents

Description

(54) PRODUCTION OF METAL ARTICLES FROM TOOL STEEL OR ALLOY STEEL POWDER (71) We, POWDREX LIMITED, a British Company of 20 Copthall Avenue, London EC2R 7JN, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following state ment:- This invention relates to the production of metal articles and is concerned with articles of tool steel and alloy steels made from steel in powder or particulate form (hereinafter called "powder").

According to previous practice metal powder is first cold compacted in a mechanically operated or isostatic press to form a "green" compact in which the powder particles are mechanically locked together and in which the density of the compact is significantly less than the true density of the solid material from which it is made. The compact thus contains voids which typically constitute between 10% and 40% of its total volume. After forming the green compact can be sintered in a controlled atmosphere or under vacuum to bond the particles together chemically and metallurgically.

A problem with existing practice is that for certain alloy materials, in particular high speed tool steels, the sintering temperature necessary to achieve a reasonable sintered bond and densification of a compact is so high that excessive growth of metal grain size and carbides may occur.

The enlargement of metal grain size and the growth and change in form of the carbides in such alloy materials can lead to low impact strength of the resulting metal article.

The present invention provides a method of making a metal article which comprises compacting tool steel or alloy steel powder to a compact of at least sixty per cent relative density, and heating the compact in a vacuum in at least three stages, in a first stage at a temperature in the range of 1070"C to 1150at to reduce the oxygen level, in a second stage for a period of half to three hours at a soak temperature above 1100"C but below the solidus temperature of the particular powder composition, and in a third stage raising the temperature of the compact to at least 10 C above the stage two soak temperature to a temperature above the solidus temperature of the powder composition and maintaining it in this range for from five seconds to fifteen minutes whereby the density is increased to at least ninety six per cent full density.

The invention thus discloses a means for minimising grain growth and controlling carbide growth, type and distribution whilst achieving a material density in excess of 96% of the theoretical value.

The preferred method of carrying out the invention will now be described. In a deoxidation treatment process of the metal powder before compaction it is treated by placing it in shallow trays in a vacuum furnace heating to between 800"C and 1 1000C for a period of half to three hours.

The temperature selected is the highest possible which will not so agglomerate the metal powder that treatment in a hammer mill or other apparatus to break up lumps significantly affects the characteristics of metal particle hardness, particle shape etc.

Typically for tool steels a temperature of 950"C is used. In practice, however, it has been found that providing the vacuum level is held below 10-2 Torr and providing the depth of the powder in the trays does not exceed 3 centimetres, high speed tool steel containing levels of carbon between 0.75% and 1.5% and with an initial typical oxygen content of 1500 - 2000 p.p.m. may be deoxidised to a level of 200 - 700 p.p.m.

by this treatment. Part of the carbon in the steel serves to combine with the oxygen to release gaseous oxides of carbon which are removed in the vacuum pumping system.

In practice it has been found that if the depth of powder in the trays exceeds 3 centimetres, full deoxidation throughout the powder may not be achieved. It is however recognised that means of ventilating a deeper bed could readily be engineered to achieve the same result.

The powder so treated for deoxidation is then formed into green compacts of density 60 to 90% full density, care being taken to avoid reoxidation or entry of moisture into the compact during handling or in storage. Lubricant may be added to the powder if desired before it is pressed.

The green compacts are sintered in a vacuum furnace in the following stages.

The temperatures over 1100"C quoted below apply to high speed tool steels whose final sintered carbon content is in the range 0.82 ,b - 0.88%. Lower temperatures should be used for higher carbon contents and a linear relationship applies between carbon content and sintering temperature from 0.7% to 1.2% carbon.

In an operational preliminary stage the compact may if necessary be preheated to between 250"C and 450"C for at least half an hour for removal of any lubricants used during the pressing operation.

In a first subsequent stage of treatment the temperature of the furnace is raised to between 1070"C and 1150"C for a period of at least half an hour and normally an hour, during which the compact is further deoxidised to a level of between 20 and 200 p.p.m. preferably at a vacuum level of 10-2 to 10-4 Torr. In this stage sintering is inititiated with some bonding and rounding of the powder particles. If the powder has a higher oxygen content than 700 p.p.m. then the heating should be continued to reduce it to below 700 p.p.m.

In a second stage of treatment the compact is heated to a pre-sintering temperature at which further bonding occurs between the individual particles of powder and partial densification of the compact is achieved.

This process is allowed to continue for a period of between half to three hours at a temperature selected to avoid significant growth of grains or carbides for the particular alloy being treated. The temperature preferably should be in the range 1100 1200"C: for high speed - tool steels a temperature in the range 1145 - 1200"C is preferred. In this second stage heating the temperature is below the solidus temperature of the powder composition.

In a third stage of treatment the compact is raised in temperature rapidly to at least 10 C above and preferably 10 - 40"C above the stage two soak temperature, at which higher temperature some liquid phase forms. The material is then held under vacuum for from 5 seconds to 15 minutes at this higher temperature. By this treatment the compact is substantially made solid (that is at least 96% density}. Providing the time and temperature at which the compact is held at the stage three temperature (typically 1205"C for high speed tool steel) are limited to below that necessary to cause grain growth, the resulting metal will remain of fine grain structure and fine even carbide distribution, yet be made substantially fully dense since densification in this third stage liquid phase sintering process is found to be relatively rapid compared with grain and carbide growth.

Whilst the above example has been quoted for powders of oxygen contents below 700 p.p.m., we have found that it is possible to achieve similar densities and microstructures on powders of higher oxygen contents, but consistent results are harder to obtain. It is not, however, possible to obtain high quality results on powders with oxygen contents in excess of 2500 p.p.m.

using this process.

The essential difference between this method of treatment of compacts to form metal articles and previous practice is in that the sintering step to initiate a bond is differentiated from the final solidification step to densities above 96%. Thus solidification at high temperature is carried out for a short time whilst bonding by sintering is achieved for a relatively long period at a relatively low temperature. No external pressure is required during sintering.

WHAT WE CLAIM IS: 1. A method of making a metal article which comprises compacting tool steel or alloy steel powder to a compact of at least sixty per cent relative density, and heating the compact in a vacuum in at least three stages, in a first stage at a temperature in the range of 1070"C to 1150"C to reduce the oxygen level, in a second stage for a period of half to three hours at a soak temperature above 1l000C but below the solidus temperature of the particulate powder composition, and in a third stage raising the temperature of the compact to at least 10 C above the stage two soak temperature to a temperature above the solidus temperature of the powder composition and maintaining it in this range for from five second to fifteen minutes whereby the density is increased to at least ninety six per cent full density.

2. A method according to Claim 1 including an initial heating stage preceding said three heating stages in which the compact is heated between 250"C and 450"C to substantially remove lubricants.

3. A method according to Claim 1 or Claim 2 in which said first heating stage

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. release gaseous oxides of carbon which are removed in the vacuum pumping system. In practice it has been found that if the depth of powder in the trays exceeds 3 centimetres, full deoxidation throughout the powder may not be achieved. It is however recognised that means of ventilating a deeper bed could readily be engineered to achieve the same result. The powder so treated for deoxidation is then formed into green compacts of density 60 to 90% full density, care being taken to avoid reoxidation or entry of moisture into the compact during handling or in storage. Lubricant may be added to the powder if desired before it is pressed. The green compacts are sintered in a vacuum furnace in the following stages. The temperatures over 1100"C quoted below apply to high speed tool steels whose final sintered carbon content is in the range 0.82 ,b - 0.88%. Lower temperatures should be used for higher carbon contents and a linear relationship applies between carbon content and sintering temperature from 0.7% to 1.2% carbon. In an operational preliminary stage the compact may if necessary be preheated to between 250"C and 450"C for at least half an hour for removal of any lubricants used during the pressing operation. In a first subsequent stage of treatment the temperature of the furnace is raised to between 1070"C and 1150"C for a period of at least half an hour and normally an hour, during which the compact is further deoxidised to a level of between 20 and 200 p.p.m. preferably at a vacuum level of 10-2 to 10-4 Torr. In this stage sintering is inititiated with some bonding and rounding of the powder particles. If the powder has a higher oxygen content than 700 p.p.m. then the heating should be continued to reduce it to below 700 p.p.m. In a second stage of treatment the compact is heated to a pre-sintering temperature at which further bonding occurs between the individual particles of powder and partial densification of the compact is achieved. This process is allowed to continue for a period of between half to three hours at a temperature selected to avoid significant growth of grains or carbides for the particular alloy being treated. The temperature preferably should be in the range 1100 1200"C: for high speed - tool steels a temperature in the range 1145 - 1200"C is preferred. In this second stage heating the temperature is below the solidus temperature of the powder composition. In a third stage of treatment the compact is raised in temperature rapidly to at least 10 C above and preferably 10 - 40"C above the stage two soak temperature, at which higher temperature some liquid phase forms. The material is then held under vacuum for from 5 seconds to 15 minutes at this higher temperature. By this treatment the compact is substantially made solid (that is at least 96% density}. Providing the time and temperature at which the compact is held at the stage three temperature (typically 1205"C for high speed tool steel) are limited to below that necessary to cause grain growth, the resulting metal will remain of fine grain structure and fine even carbide distribution, yet be made substantially fully dense since densification in this third stage liquid phase sintering process is found to be relatively rapid compared with grain and carbide growth. Whilst the above example has been quoted for powders of oxygen contents below 700 p.p.m., we have found that it is possible to achieve similar densities and microstructures on powders of higher oxygen contents, but consistent results are harder to obtain. It is not, however, possible to obtain high quality results on powders with oxygen contents in excess of 2500 p.p.m. using this process. The essential difference between this method of treatment of compacts to form metal articles and previous practice is in that the sintering step to initiate a bond is differentiated from the final solidification step to densities above 96%. Thus solidification at high temperature is carried out for a short time whilst bonding by sintering is achieved for a relatively long period at a relatively low temperature. No external pressure is required during sintering. WHAT WE CLAIM IS:

1. A method of making a metal article which comprises compacting tool steel or alloy steel powder to a compact of at least sixty per cent relative density, and heating the compact in a vacuum in at least three stages, in a first stage at a temperature in the range of 1070"C to 1150"C to reduce the oxygen level, in a second stage for a period of half to three hours at a soak temperature above 1l000C but below the solidus temperature of the particulate powder composition, and in a third stage raising the temperature of the compact to at least 10 C above the stage two soak temperature to a temperature above the solidus temperature of the powder composition and maintaining it in this range for from five second to fifteen minutes whereby the density is increased to at least ninety six per cent full density.

2. A method according to Claim 1 including an initial heating stage preceding said three heating stages in which the compact is heated between 250"C and 450"C to substantially remove lubricants.

3. A method according to Claim 1 or Claim 2 in which said first heating stage

takes at least half an hour.

4. A method according to any one of Claims 1 to 3 in which in said first heating stage the oxygen content of the powder is reduced to below 700 p.p.m.

5. A method according to Claim 4 in which said oxygen content is reduced to below 200 p.p.m.

6. A method according to any of Claims 1 to 5 in which the powder is a tool steel powder and the temperature range in the second stage is 1145 to 1200"C.

7. A method according to any of Claims 1 to 6 in which before compaction the powder is heated in a vacuum in the temperature range 800" to 1100 C for half an hour to three hours to reduce the oxygen content of the powder.

8. A method according to Claim 7 in which the vacuum is better than 10-2 Torr.

9. A method according to Claim 7 or Claim 8 in which the oxygen content is reduced below 700 parts per million.

10. A method of making a metal article from tool steel or alloy steel powder according to Claim 1 and substantially as described herein.

11. An article made by the method of any of Claims 1 to

12.