IE42143B1 - Superhard martensite and method of making the same - Google Patents

Abstract

1483891 Ion implantation; hardening steel surfaces N N ENGEL 15 Nov 1974 [14 Dec 1973] 49506/74 Heading C7U A steel substrate containing 0.3 to 1.8% of interstitial alloying atoms is implanted with ions of an element which is insoluble in iron by ionbombardment, and then heat-treated to produce a superhard martensitic structure. The substrate may initially contain Be, B, C or N as interstitial alloying atoms, or these may be alloyed therewith after ion-bombardment. The steel may be cleaned before placing in a vacuum chamber as the cathode, and the chamber is flushed with inert gas prior to bombarding with ions of He, Ne, Ar, Kr, Xe, Rn, Li, Na, K, Rb, Cs, Fr, Ca, Sr, Ba, Ra, Ag, Cd, Hg, Tl, Pb, Bi, Mg, Y, La, Zr, Hf, Th, Ta, Cu, In, Se, Te or Po; preferred ions are those of size similar to Fe. The steel is then heated into the austenite range and quenched in water, oil, or air to produce a superhard martensite, e.g. of Knoop hardness above 1000, and of maximum grain size 0.001 mm.

Description

This invention relates to an ion implanted hardened steel and more particularly to a superhard martensite and method of making the same.

Various techniques have been employed to coat the surface of a substrate with a material.including ion deposition. An apparatus employed by me for such ion planting as hereinafter described is described in NASA Technical Note D-2707, Deposition of Thin Films by Ion Planting on Surfaces Having Various Configurations, by T Spalvins, et al, November 1965.

A steel substrate has a cubic body-centered lattice. When the substrate is heated, the configuration changes to a cubic face-centered lattice (austenite) which, when quenched, forms a tetragonal body-centered lattice. The tetragonal form is martensite.

By nucleating and/or inhibiting the growth of the martensite grain in the hardening process, a super-fine grain will be obtained. The finer the grain in the martensite the harder will be the steel. Simultaneously insoluble embedded atoms act as barriers for the movement of dislocations contributing further to hardness and strength.

Briefly described, the growth of martensite grains is inhibited by ion implanting in the substrate (steel matrix) a sufficient amount of any element which is insoluble in iron. These elements include helium, neon, argon, krypton, xenon, radon, lithium, sodium, potassium, rubidium, cesium, francium, calcium, strontium, barium, radium, silver, cadmium, mercury, thallium, lead and bismuth. The following additional elements can be utilized in the present method as they all possess a marked low solubility in iron or have a solubility limit: beryllium, magnesium, yttrium, lanthamum, zirconium, hafnium, thorium, tantalum, copper, indium, selenium, tellurium, and polonium. This process produces an exceptionally hard martensite useful in producing excellent cutting tools with wear resistant surfaces - it also can be used for example in gear wheels, ball bearings, or measuring tools. The fine-grained martensite also provides improved fatigue and impact strength useful for example in springs or hammers.

According to a first aspect of the present invention I provide a method of treating a steel substrate, containing from 0.3 to 1.8% by weight of interstitial alloying atoms, in order to produce a superhardened martensite structure comprising the steps of implanting ions of an element which is substantially insoluble in iron into the substrate by ion bombardment and then heat treating the substrate to produce the hardened martensiticstructure. Preferably the interstitial alloying atoms are selected from beryllium,boron, carbon or nitrogen and the alloying may be effected after the ion bombardment.

The invention includes a preferred method wherein the ion implanting step includes placing said substrate in a vacuum, admitting an inert gas into said vacuum, producing an electrical plasma discharge through said inert gas with said substrate as the cathode and maintaining the potential with vacuum in the range which will support the plasma.

The invention also includes according to a second aspect a superhard martensite prepared by the method according to the first aspect of the invention wherein the implanted element is helium, neon, argon, krypton, xenon, radon, lithium, sodium, potassium, rubidium, cesium, francium, calcium,strontium, barium, radium, silver, cadmium, mercury, thallium, lead, bismuth, beryllium, yttrium, lanthanum, zirconium, hafnium, thorium, tantalum, copper, indium, selenium, tellurium or polonium.

The superhard martensite may comprise a steel substrate in which the surface thereof contains an element insoluble in iron embedded in the iron, the surface exhibiting a grain size smaller than 0.001 mm as the longest dimension after normal quenching.

It has been found that the present method works best on normalized or spheroidized (annealed) steel, i.e. steel of low hardness.

A substrate for. use in the present invention should be a steel with sufficient interstitial alloying atoms therein to be hardenable, i.e. an alloy content ranging from 0.3$ to 1.8% by weight with the optimum range being from 0.5$ to 1.0$ by weight. Such interstitial alloying atoms are from the second period of the periodic table and comprise beryllium, boron, carbon and nitrogen. Substitutional alloying elements found within the steel substrate are generally of no significance in the present invention.

It has been found that an element which is insoluble in iron, when implanted into a steel substrate, will retard the growth of or/and nucleate the grain during martensite formation, thereby producing a more uniform and finer grained martensite structure, resulting in a harder substrate.

To treat, according to a preferred method of carrying out the present invention, a hardenable steel substrate or object (0.6% carbon, for example) the steel should first be cleaned on its surface. This is accomplished by any conventional cleaning method. The substrate is then placed inside a vacuum chamber on a suitably supported and insulated metallic plate to the cathode, to which one terminal of a high potential d.c. current source is connected. Larger objects may be placed on insulators and directly connected to the negative side of the d.c. source.

The other terminal of the d.c. source is connected to a suitable anode which may be the conducting metallic base of the chamber. More often the substrate is placed at the bottom of the chamber and the anode above it to make it easier to load and unload the chamber.

The chamber is next evacuated to a pressure of about 1 x 10 mm Hg. It is cyclically flushed with Argon or whatever inert gas is to be used or to be implanted fh the substrate and evacuated, two or three times.

Argon or other gas is then slowly let into the chamber with simultaneous application of potential between the substrate or object and the anode. A pink plasma starts forming around 500-800 volts at a -5 vacuum of about 2.0 x 10 mm Hg. The potential is then increased to any desired value, such as 4,5. KV. The object is thus bombarded for a specific time (2 to 4 minutes) with this Argon or inert gas plasma.

It is then cooled inside the chamber to prevent any oxidation.

When a solid is further ion plated for the formation of a wear resistant corrosion resistant or other purpose coat, the other terminal is connected to a tungsten wire anode which is then resistance heated to melt the coating material. An electron gun evaporator or other vapour source may be used as the anode.

The object or substrate is subsequently heated up into the austenite temperature range and quenched to martensite in water, oil or air depending on the alloy content. The treated surface layer may also be heated into the austenite range by ion bombardment and quenched by the backing substrate material as a heat sink or by cooled contact holders. Helium, argon or other gases may also be let into the vacuum for quenching.

The voltage applied to the system can vary for example from 200 volts to 20,000 volts. Ion accelerators can also be employed which utilize up to 2,000,000 volts.

It has been found that a voltage of 4 KV applied for a few minutes will cause ion penetration of argon into the substrate of approximately microns. The implanting concentration fades out after 20 microns when 4 KV is applied.

The speed of the impinging ions will determine the depth of penetration. The higher the applied potential jjpd the lower the gas pressure, thfe faster will the ions move when impinging on the substrate. The distribution of insoluble implanted atoms is controlled by the hardness of the substrate and the history of potential and pressure applied during the implantation time.

In some instances the plasma can be better maintained and working i conditions extended to pressures and/or potentials which could not otherwise be used, if a magnetic field, high frequency or radio frequency or radiation is applied to the plasma causing a further ionization of the gas beyond that caused by the static dc bias. Such methods are used often to increase ionization in plasmas.

Basically, any element which is insoluble in iron can be utilized in the present invention including the inert gases: helium, neon, argon, krypton, xenon, radon; the alkaline metals: lithium, sodium, potassium, rubidium, cesium, francium; the alkaline earths: calcium, strontium, barium, radium; plus the insoluble metals: silver, cadmium, mercury, . thallium, lead and bismuth. The following elements can also be effectively ion implanted into steel substrate as they have either a marked low or limited solubility limit in iron: beryllium (0.1% by weight), magnesium (0.1% in weight), yttrium (low), lanthanum (0.1% by weight), zirconium (low), hafnium (low), thorium (low), tantalum (low), copper (low), indium (low), selenium (low), tellurium (low) and polonium (low).

Although all of the above identified elements can be employed in this implantation procedure, the preferred elements are those with an atomic size which is comparable to that of iron. This is best illustrated by exaniin- 6 42143 ing the effectiveness of the inert gases in this process. In going down the list of these gases on the periodic table, it is found that helium is next to the lowest in effectiveness, neon is more effective, argon is the most effective, krypton is comparable to neon and xenon is the least effective. Argon is the most effective because its atomic size is about the same as iron; xenon and neon have atomic sizes which are too large and too small respectively, as compared to iron.

The present method could also be performed by simultaneously or successively bombarding the substrate with one of the selected implantation elements and one of the selected interstitial alloy elements and then hardening. This procedure would produce the same result, namely, a superhard martensite. The present invention could also be performed by bombarding a mild steel with insoluble ions and carbonizing the steel by one of the conventional methods either before or after the ion bombard15 ment, to obtain a core hardened product with superhard surface.

The following table I is illustrative of the process of the present invention. The steel substrates were ion implanted with various elements at various potentials for a selected time period and then hardened. The resultant product was measured for hardness. The Knoop hardness indenta20 tions were made with a 100 gram load and measured at twenty (20),times magnification. - 7 42143 TABLE I El ement Untreated Substrate Argon Argon Argon Xenon Xenon Helium Helium Silver Voltage (kv) Time (minutes) Hardness (Knoop) 4.5 3 830 1080 4.5 7 1000-1030 2.5 5 1050-1110 4.5 5 910 4.5 10 1000 2.5 5 1000 2.5 2 890-910 3.0 (plus 3 mins, of silver ion implan ting) 3 960 The following table illustrates the Knoop hardness obtained when elements (iron and titanium) which are soluble in iron are implanted into a steel substrate for a selected time period and then hardened: TABLE II Element Voltage (kv) Time (minutes) Hardness (Knoop) Iron 3.0 3 810 Titanium 3.0 3 840 Helium and Iron He 2.5 2 790-810 Fe 2.5 3 In the examples of Table I, the steel substrates, employed, contained carbon by weight, and the remainder iron. Each substrate was approxi irately 2 inches by | inch by 1/32 inch.

A chamber, similar to that described in the aforesaid NASA technical note D-2707, was employed, being first evacuated several times with the gas to be employed and then evacuated to a vacuum at which the plasma - 8 42143 could be sustained, namely in the neighbourhood of 5 X 10 (2 - 50 X ) millimeters of mercury.

In the hardening step, each substrate, after being implanted with ions by bombardment in the chamber, was heated to from 850°C to about 1050°C, preferably to about 1000°C so as to be in the gamma austenite range and then quenched in water, at about room temperature to produce martensite.

Thereafter, each sample substrate was etched on a surface using a 2 solution of Nital (nitric acid and ethanol). Intead of the usual accicular structure of martensite, which in the untreated sample had crystals the major length of which was about 40 mm at 6700 times magnification, the treated substrates exhibit a very fine grained martensite with grains no longer than 6 mm. Since the volume and weight of the grains are proportional to the third power of this length or diameter the treated structure has 4q3 =» 305 times as many grains and 50 times as much grain boundary area.

Claims (15)

1. A method of treating a steel substrate containing from 0.3 to 1.8$ by weight of interstitial alloying atoms in order to form a superhardened martensite-structure comprising the steps of implanting ions of an element which is substantially insoluble in iron into the substrate by ion bombardment and then heat treating the substrate to produce the hardened martensiticstructure.

2. A method as claimed in Claim 1, wherein the substrate contains interstitial alloying atoms of beryllium, boron, carbon or nitrogen.

3. A method as claimed in Claim 1 wherein the substrate is alloyed with beryllium, boron, carbon or nitrogen after the ion bombardment.

4. A method as claimed in Claim 1 wherein the amount of interstitial alloy in the steel substrate ranges from 0.5$ to 1.0$ by weight.

5. A method as claimed in Claim 1 wherein the implanted element is helium, neon, argon, krypton, zenon, radon, lithium, sodium, potassium, rubidium, cesium,francium, calcium, strontium, barium, radium, silver, cadmium, mercury, thallium, lead, bismuth, beryllium, magnesium, yttrium, lanthanum, zirconium, hafnium, thorium, tantalum, copper, indium, selenium, tellurium or polonium.

6. A method as claimed in Claim 1 wherein the implanted element has an atomic size substantially like that of iron.

7. A method as claimed in Claim 6 wherein the element is argon or silver.

8. A substrate treated in accordance with any of Claims 1 to 7 to produce a superhard martensite.

9. A method as claimed in Claim 1 wherein the ion implanting step includes placing said substrate in a vacuum, admitting an inert gas into said vacuum, producing an electrical plasma discharge through said inert gas with said substrate as the cathode and maintaining the potential with vacuum in the range which will support the plasma. - 10 42143

10. A superhard martensite prepared by the method claimed in Claim 1 wherein the element is helium, neon, argon, krypton, xenon, radon, lithium, sodium, potassium, rubidium, cesium,francium, calcium, strontium, barium, radium, silver, cadmium, mercury, thallium, lead, bismuth, beryl5 lium, yttrium, lanthanum, zirconium, hafnium, thorium, tantalum, copper, indium, selenium, tellurim or polonium.

11. The superhard martensite according to Claim 10 wherein said surface has a Knoop hardness above 1000.

12. A superhard martensite according to Claim 11 comprising a steel 10 substrate in which the surface thereof contains an element insoluble in iron embedded in the iron, the surface exhibiting a grain size smaller than fl.001 mm as the longest dimension after normal quenching.

13. A method as claimed in Claim 1 where the plasma is further ionized by a magnetic field, high frequency or radio frequency, or by radiation beyond 15 the ionization caused by the static d.c. bias.

14. A method of superhardening a substrate containing iron substantially as hereinbefore described.

15. A steel substrate comprising a superhard martensite substantially as hereinbefore described.