Patent Description:
It is required that cylinders used for machines for injection-molding or extrusion-molding plastics, etc. withstand wear by resins, additives, etc. during high-temperature molding, and can be produced at lower cost. To meet such requirements, a molding machine cylinder having a bimetal structure comprising a lining layer having tungsten carbide particles dispersed in a nickel alloy matrix, which is formed by a centrifugal casting method on an inner surface of a steel-made circular cylinder, has conventionally been used.

In recent extrusion or injection molding of reinforced plastics and flame-retardant plastics used in electronic or electric parts and automobile parts, molding machine cylinders are exposed to severe wear and corrosion. For example, because the reinforced plastics exert severe wear by their glass fibers or inorganic fillers, and the flame-retardant plastics exert severe corrosion by isolated halogens, development has been conducted to provide molding machine cylinders having lining layers formed by HIP.

<CIT> discloses a composite cylinder for high-temperature, high-pressure molding, which comprises a cylinder body of heat-resistant martensitic steel, and a lining layer of a Ni-based alloy containing tungsten carbide particles, the lining layer being formed by uniformly dispersing <NUM>-<NUM> parts by weight [<NUM>-<NUM>% by volume (converted by calculation using that the Ni-based alloy has a density of <NUM>/cm<NUM>, and the tungsten carbide has a density of <NUM>/cm<NUM>, the same is true below)] of tungsten carbide particles having particle sizes of <NUM>-<NUM> in <NUM> parts by weight of atomized Ni-based alloy powder having wear resistance and corrosion resistance, and pressure-sintering the lining layer on the inner surface of the cylinder body by HIP.

<CIT> discloses a cylinder having excellent wear resistance and corrosion resistance, which has a two-layer structure in which an inner layer of a sintered Ni-based alloy containing <NUM>-<NUM>% by mass [<NUM>-<NUM>% by volume (converted)] of tungsten carbide particles is integrally fused to an inner surface of a metal cylinder as an outer layer by HIP. <CIT> describes that the tungsten carbide particles preferably have particle sizes of <NUM>-<NUM> from the aspect of surface roughening resistance.

However, even the molding machine cylinders having lining layers formed by HIP, which are described in <CIT> and <CIT>, do not have sufficient wear resistance and corrosion resistance for the molding of reinforced plastics, flame-retardant plastics, etc. Further improvements are thus desired. For example, if the percentage of tungsten carbide particles in the lining layer were increased to improve wear resistance and corrosion resistance, the lining layer would have decreased mechanical strength when exceeding <NUM> parts by weight [<NUM>% by volume (converted)] per <NUM> parts by weight of the Ni-based alloy, as described in the specification of <CIT>. Accordingly, it is difficult to obtain further improved wear resistance and corrosion resistance by increasing the amount of tungsten carbide particles. In addition, hard tungsten carbide particles in the lining layer necessitate long time for machining for assembling in a molding machine. Document <CIT> discloses a lining material casted on an inner peripheral surface of a cylinder by a centrifugal casting method, the outer layer comprising tungsten carbide particles in a Ni-based alloy, wherein the inner layer has <NUM>% to <NUM>% of the thickness of the lining material.

Accordingly, an object of the present invention is to provide a molding machine cylinder, according to claim <NUM>, comprising a lining layer having excellent wear resistance, corrosion resistance and machinability and suffering no deterioration of mechanical strength even when the amount of tungsten carbide particles is increased, and its production method, according to claim <NUM>.

In view of the above object, the inventors have conducted intensive research on a HIPed lining layer of a Ni-based alloy containing tungsten carbide particles, finding that when the structure of the lining layer is optimized, the lining layer can be provided with improved wear resistance and corrosion resistance as well as excellent machinability, without suffering the deterioration of mechanical strength even when the percentage of tungsten carbide particles is increased. The present invention has been completed based on such finding.

Thus, the molding machine cylinder of the present invention comprises a HIP-sintered lining layer formed on an inner surface of a cylindrical steel body, the lining layer comprising <NUM>-<NUM>% by volume of tungsten carbide particles having a median diameter d<NUM> of <NUM>-<NUM> and a matrix composed of an Ni-based alloy, and the maximum length of the matrix in an arbitrary cross section being <NUM> or less.

The tungsten carbide particles preferably have d<NUM> of <NUM>-<NUM> and d<NUM> of <NUM>-<NUM>, wherein d<NUM> and d<NUM> are particle sizes at cumulative volumes corresponding to <NUM>% and <NUM>%, respectively, of the total volume, in a curve expressing the relation between the particle size and cumulative volume (value obtained by accumulating the volume of particles having sizes up to a particular size) of tungsten carbide particles.

The matrix composed of an Ni-based alloy preferably contains by mass <NUM>-<NUM>% of Si, <NUM>-<NUM>% of Cr, and <NUM>-<NUM>% of B.

The matrix composed of an Ni-based alloy preferably contains at least one of less than <NUM>% of C, <NUM>% or less of Mn, <NUM>% or less of Co, <NUM>% or less of Fe, and <NUM>% or less of Mo.

The lining layer preferably contains <NUM>-<NUM>% by volume of the tungsten carbide particles.

The method of the present invention for producing a cylinder for a molding machine comprises the steps of.

Because the molding machine cylinder of the present invention comprises a lining layer having excellent wear resistance, corrosion resistance and machinability and suffering no deterioration of mechanical strength even when the amount of tungsten carbide particles is increased, it is suitable for injection-molding or extrusion-molding machines for plastics.

As shown in <FIG>, the cylinder <NUM> for a molding machine according to the present invention comprises a HIP-sintered lining layer, which is simply called lining layer <NUM>, on an inner surface of a cylindrical steel body <NUM>, the lining layer <NUM> comprising <NUM>-<NUM>% by volume of tungsten carbide particles having a median diameter d<NUM> of <NUM>-<NUM> and a matrix composed of an Ni-based alloy, and the maximum length of the matrix in an arbitrary cross section being <NUM> or less. The lining layer <NUM> is formed by sintering a powdery mixture of Ni-based alloy powder and tungsten carbide powder by a HIP process, such that it is integrally fused to an inner surface of the cylindrical body <NUM>.

As shown in <FIG>, for example, the lining layer <NUM> obtained by HIP-sintering Ni-based alloy powder and tungsten carbide powder has a structure in which gaps between tungsten carbide particles (white portions) <NUM> are filled with a matrix composed of an Ni-based alloy <NUM> (black portions). In the figure, there are gray portions (boundary phases <NUM>) adjacent to the tungsten carbide particles <NUM>. The boundary phases <NUM> having an intermediate composition between those of the Ni-based alloy <NUM> and the tungsten carbide particles <NUM> are presumably formed by their reaction. The lining layer <NUM> in the molding machine cylinder of the present invention should have a structure in which "the maximum length of the matrix" determined on a SEM photograph of its cross section by a method described below is <NUM> or less. When the maximum length of the matrix is more than <NUM>, as described above, the lining layer <NUM> has low mechanical strength as well as poor wear resistance and corrosion resistance. The maximum length of the matrix is preferably <NUM> or less, further preferably <NUM> or less, most preferably <NUM> or less.

As shown in <FIG>, "the maximum length of the matrix" is determined by drawing two diagonal lines on a SEM photograph of each field (field area: <NUM> × <NUM>) taken at a magnification of about <NUM> times (acceleration voltage: <NUM> kV), measuring the lengths of line segments of each diagonal line crossing matrix regions composed of the Ni-based alloy <NUM> with no tungsten carbide particles <NUM> and no boundary phases <NUM>, selecting the maximum length in each field, and averaging the maximum lengths in <NUM> fields. For example, in <FIG> schematically showing the SEM photograph of Comparative Example <NUM>, the longest line segment "a" among line segments "a" to "f" (only six long line segments are shown in the figure), in which two diagonal lines cross matrix regions, is selected, and the longest line segments in <NUM> fields are averaged to determine the maximum length of the matrix.

Comprising <NUM>-<NUM>% by volume of tungsten carbide particles having a median diameter d<NUM> of <NUM>-<NUM> and the matrix composed of the Ni-based alloy, with a structure in which "the maximum length of the matrix" is <NUM> or less, the lining layer of the present invention exhibits excellent wear resistance, corrosion resistance and machinability.

Comprising <NUM>-<NUM>% by volume of relatively small tungsten carbide particles <NUM> having a median diameter d<NUM> of <NUM>-<NUM> and the matrix composed of the Ni-based alloy <NUM>, the lining layer <NUM> has excellent machinability as well as improved wear resistance.

The percentage of the tungsten carbide particles <NUM> is <NUM>-<NUM>% by volume per the total amount of the Ni-based alloy <NUM> and the tungsten carbide particles <NUM> constituting the lining layer <NUM>. Less than <NUM>% by volume of tungsten carbide particles <NUM> insufficiently improves the wear resistance, while more than <NUM>% by volume of tungsten carbide particles <NUM> undesirably decreases the mechanical strength. The percentage of the tungsten carbide particles <NUM> is preferably <NUM>% or more by volume, more preferably <NUM>% or more by volume. Also, the percentage of the tungsten carbide particles <NUM> is preferably <NUM>% or less by volume, more preferably <NUM>% or less by volume.

The tungsten carbide particles <NUM> has a median diameter d<NUM> of <NUM>-<NUM>. Uniform dispersion is difficult when it is less than <NUM>, while the lining layer has poor machinability when it is more than <NUM>. The median diameter d<NUM> of the tungsten carbide particles <NUM> is preferably <NUM> or more and <NUM> or less. Because the lining layer is densely filled with interconnected tungsten carbide particles, it is difficult to determine the sizes of WC particles on an electron photomicrograph. Accordingly, used herein as the median diameter d<NUM> of the tungsten carbide particles <NUM> dispersed in the lining layer is that measured on tungsten carbide powder as a raw material by a Microtrac particle size distribution meter <NUM>-X100 available from Nikkiso Co. , for example. Because the lining layer in the molding machine cylinder of the present invention is sintered at a relatively low temperature (<NUM>-<NUM>) by HIP, the particle sizes of tungsten carbide powder as a raw material are substantially not different from those of the tungsten carbide particles in the lining layer. Accordingly, the median diameter d<NUM> of the tungsten carbide particles dispersed in the lining layer may be expressed by the median diameter d<NUM> of the tungsten carbide powder as a raw material.

The tungsten carbide particles preferably have d<NUM> of <NUM>-<NUM>, and d<NUM> of <NUM>-<NUM>, wherein d<NUM> and d<NUM> are particle sizes at cumulative volumes corresponding to <NUM>% and <NUM>%, respectively, of the total volume, in a curve expressing the relation between the particle size and cumulative volume (value obtained by accumulating the volume of particles having sizes up to a particular size) of tungsten carbide particles.

The matrix composed of the Ni-based alloy <NUM> constituting the lining layer <NUM> preferably comprises by mass <NUM>-<NUM>% of Si, <NUM>-<NUM>% of Cr, <NUM>-<NUM>% of B, <NUM>% or less of Mn, and <NUM>% or less of Co as indispensable components, the balance being Ni and inevitable impurities.

Si is dissolved in the matrix composed of the Ni-based alloy <NUM> in the lining layer <NUM> to increase its hardness, thereby contributing to improvement in wear resistance. Less than <NUM>% by mass of Si does not sufficiently provide this effect, while more than <NUM>% by mass of Si makes the lining layer <NUM> brittle. The Si content is more preferably <NUM>% or more by mass, further preferably <NUM>% or more by mass, most preferably <NUM>% or more by mass. Also, the Si content is more preferably <NUM>% or less by mass, further preferably <NUM>% or less by mass, most preferably <NUM>% or less by mass.

Cr is mainly dissolved in the matrix composed of the Ni-based alloy <NUM> to increase the strength and corrosion resistance. Less than <NUM>% by mass of Cr unlikely provides the effect of improving the strength and corrosion resistance, while more than <NUM>% by mass of Cr reduces the toughness of the matrix. The Cr content is more preferably <NUM>% or more by mass, further preferably <NUM>% or more by mass. Also, the Cr content is more preferably <NUM>% or less by mass, further preferably <NUM>% or less by mass, most preferably <NUM>% or less by mass.

B is combined with Ni, Cr, etc. to precipitate high-hardness borides in the structure, thereby improving the hardness of the matrix. Less than <NUM>% by mass of B does not provide such effect sufficiently, while more than <NUM>% by mass ofB forms excessive borides, lowering the strength of the lining layer. The B content is more preferably <NUM>% or more by mass, further preferably <NUM>% or more by mass. Also, the B content is more preferably <NUM>% or less by mass, further preferably <NUM>% or less by mass.

Mn removes undesirable substance such as oxides, etc. in the production of the Ni-based alloy by an atomizing method. More than <NUM>% by mass of Mn undesirably deteriorates the corrosion resistance of the lining layer <NUM>. The Mn content is more preferably <NUM>% or more by mass, further preferably <NUM>% or more by mass. Also, the Mn content is more preferably <NUM>% or less by mass, further preferably <NUM>% or less by mass.

Like Ni, Co imparts corrosion resistance to the lining layer <NUM>, and is dissolved in the matrix to improve its strength. More than <NUM>% by mass of Co is not economical because of the saturation of such effect. The Co content is more preferably <NUM>% or more by mass, further preferably <NUM>% or more by mass. Also, the Co content is more preferably <NUM>% or less by mass, further preferably <NUM>% or less by mass.

The inevitable impurities include C, Fe, etc. C is preferably <NUM>% or less by mass, because more than <NUM>% by mass of C provides brittleness and low strength to the lining layer. Fe is preferably <NUM>% or less by mass, because more than <NUM>% by mass of Fe provides low corrosion resistance.

The cylindrical body is preferably formed by high-yield-strength steel, to hold the lining layer strongly and prevent the cracking of the lining layer during molding. Such high-yield-strength steel is preferably carbon steel or microalloyed steel, particularly microalloyed steel.

The carbon steel per se may be known one, and, for example, carbon steel containing <NUM>-<NUM>% by mass of C is preferable. The general composition of such carbon steel comprises <NUM>-<NUM>% by mass of C, <NUM>-<NUM>% by mass of Si, and <NUM>-<NUM>% by mass of Mn, the balance being substantially Fe and inevitable impurities.

Because the microalloyed steel containing alloying elements such as V, etc. has excellent yield strength and toughness without heat treatment (tempering), it can be formed into a composite cylinder at lower cost than steel such as S45C, SCM440, etc. (needing a heat treatment after HIP to obtain sufficient yield strength).

The microalloyed steel for forming a cylindrical body generally has a composition comprising <NUM>-<NUM>% by mass of C, <NUM>-<NUM>% by mass of Si, <NUM>-<NUM>% by mass of Mn, and <NUM>-<NUM>% by mass of V, the balance being substantially Fe and inevitable impurities. The microalloyed steel may further contain as optional elements at least one selected from the group consisting of <NUM>-<NUM>% by mass of Cr, <NUM>-<NUM>% by mass of Cu, and <NUM>-<NUM>% by mass of Nb.

The method of the present invention for producing the molding machine cylinder comprises the steps of.

The method of the present invention can produce a molding machine cylinder having a HIP-sintered lining layer on an inner surface of a cylindrical steel body, the sintered lining layer comprising <NUM>-<NUM>% by volume of tungsten carbide particles having a median diameter d<NUM> of <NUM>-<NUM> and the matrix composed of the Ni-based alloy, and the maximum length of the matrix in an arbitrary cross section being <NUM> or less.

As shown in <FIG>, the steel mandrel <NUM> is placed inside the cylindrical steel body <NUM> to form an annular space <NUM> between the cylindrical body <NUM> and the mandrel <NUM>, in which the lining layer <NUM> of the molding machine cylinder <NUM> is molded. A lid <NUM> is welded to one-side ends of the mandrel <NUM> and the cylindrical body <NUM>. The mandrel <NUM> may be a solid or hollow cylinder. The mandrel <NUM> and the lids <NUM>, <NUM> can be formed by soft steel, etc..

The tungsten carbide powder and the Ni-based alloy powder are prepared. The tungsten carbide powder used has a median diameter d<NUM> of <NUM>-<NUM>. When the median diameter is less than <NUM>, the uniform mixing of the tungsten carbide powder with the Ni-based alloy powder is difficult, and the material cost is high. When the median diameter is more than <NUM>, the resultant lining layer has poor machinability. The median diameter d<NUM> of the tungsten carbide powder is preferably <NUM> or more and <NUM> or less. The tungsten carbide powder preferably has d<NUM> of <NUM>-<NUM> and d<NUM> of <NUM>-<NUM>. The tungsten carbide powder used has d<NUM>/(d<NUM> - d<NUM>) of <NUM> or more. The value of d<NUM>/(d<NUM> - d<NUM>) is an index indicating the particle size distribution. The d<NUM>/(d<NUM> - d<NUM>) of <NUM> or more provides a sharp particle size distribution, resulting in a lining layer excellent in both wear resistance and machinability. The d<NUM>/(d<NUM> - d<NUM>) of the tungsten carbide powder is preferably <NUM> or more, more preferably <NUM> or more.

The Ni-based alloy powder used has a matrix composition comprising by mass <NUM>-<NUM>% of Si, <NUM>-<NUM>% of Cr, <NUM>-<NUM>% of B, <NUM>% or less of Mn, and <NUM>% or less of Co as indispensable components, the balance being Ni and inevitable impurities, and a median diameter d<NUM> of <NUM>-<NUM>. When the median diameter is less than <NUM>, its uniform mixing with the tungsten carbide powder is difficult, and the material cost is high. When the median diameter is more than <NUM>, the resultant lining layer has the maximum length of the matrix exceeding <NUM>, exhibiting poor wear resistance and corrosion resistance. The Ni-based alloy powder preferably has a median diameter d<NUM> of <NUM> or less. The Ni-based alloy powder preferably has d<NUM> of <NUM>-<NUM> and d<NUM> of <NUM>-<NUM>. The Ni-based alloy powder used has d<NUM>/(d<NUM> - d<NUM>) of <NUM> or more. The value of d<NUM>/(d<NUM> - d<NUM>) is an index indicating the particle size distribution. The d<NUM>/(d<NUM> - d<NUM>) of <NUM> or more provides a sharp particle size distribution, leading to easy uniform mixing of the tungsten carbide powder having a sharp particle size distribution and the Ni-based alloy powder, thereby providing a lining layer having a structure in which the tungsten carbide particles are uniformly dispersed in the matrix composed of the Ni-based alloy. The d<NUM>/(d<NUM> - d<NUM>) of the Ni-based alloy powder is preferably <NUM> or more, more preferably <NUM> or more.

The Ni-based alloy powder is preferably produced by atomizing a molten Ni-based alloy. Because the atomized Ni-based alloy powder is substantially spherical, having good flowability, it can be uniformly mixed with the tungsten carbide powder. A gas atomizing method and a water atomizing method can be used for atomizing, and the water atomizing method is preferable.

The d<NUM> (median diameter), d<NUM> and d<NUM> of the tungsten carbide powder and the Ni-based alloy powder can be determined from their particle size distributions measured, for example, by a Microtrac particle size distribution meter <NUM>-X100 available from Nikkiso Co. The d<NUM> (median diameter), d<NUM> and d<NUM> are particle sizes at cumulative volumes corresponding to <NUM>%, <NUM>% and <NUM>%, respectively, of the total volume, in a curve expressing the relation between the particle size and cumulative volume (value obtained by accumulating the volume of particles having sizes up to a particular size) of powder.

After weighing the tungsten carbide powder and the Ni-based alloy powder to a volume ratio of <NUM>/<NUM> to <NUM>/<NUM> (<NUM>-<NUM>% by volume as a ratio of the tungsten carbide powder), they are dry-mixed. The weight of each powder is measured, and converted to a volume ratio using the specific gravities of the tungsten carbide and the Ni-based alloy. When the percentage of the tungsten carbide powder in the powdery mixture is less than <NUM>% by volume, the lining layer has insufficient wear resistance. When it is more than <NUM>% by volume, the lining layer has drastically reduced mechanical strength. The percentage of the tungsten carbide powder in the powdery mixture is preferably <NUM>% or more by volume, more preferably <NUM>% or more by volume. Also, the percentage of the tungsten carbide powder in the powdery mixture is preferably <NUM>% or less by volume, more preferably <NUM>% or less by volume.

The resultant powdery mixture 3a is charged into the space <NUM>. The charging of the alloy powder is preferably conducted while properly vibrating the cylindrical body <NUM>. After the powdery mixture 3a is charged, a lid <NUM> having an evacuating aperture <NUM> is welded to the other ends of the cylindrical body <NUM> and the mandrel <NUM>.

After the space <NUM> containing the powdery mixture 3a is evacuated, the evacuating aperture <NUM> of the lid <NUM> is sealed by welding, and HIP is conducted by a known method. HIP is preferably conducted at <NUM>-<NUM> and <NUM>-<NUM> MPa for <NUM>-<NUM> hours in an inert gas atmosphere such as Ar, etc. HIP produces a composite cylinder having a lining layer containing tungsten carbide particles in the matrix composed of the Ni-based alloy, which is integrally bonded to an inner surface of the cylindrical body. The lower limit of the HIP temperature is more preferably <NUM>, most preferably <NUM>. The upper limit is more preferably <NUM>.

After successively removing the lids <NUM>, <NUM> and the mandrel <NUM> from the HIPed composite cylinder by cutting, etc., the inner surface of the cylinder is finished. If necessary, an aperture for a hopper for charging materials is formed in the cylinder.

Because the median diameters and particle size distributions of tungsten carbide powder and Ni-based alloy powder are controlled in the molding machine cylinder thus produced, the lining layer has a structure in which tungsten carbide particles are uniformly dispersed in the matrix composed of the Ni-based alloy, exhibiting high mechanical strength, as well as excellent wear resistance, corrosion resistance and machinability.

The present invention will be explained in further detail by Examples, without intention of restricting the present invention thereto.

Ni-based alloys A and C having the chemical compositions shown in Table <NUM>-<NUM> were prepared as alloy materials for forming lining layers, and each was atomized to produce Ni-based alloy powders A and C having d<NUM> (median diameter), d<NUM> and d<NUM> shown in Table <NUM>-<NUM>. The Ni-based alloy powder A was sieved to less than <NUM> to provide Ni-based alloy powder B. Also, two types of tungsten carbide powders A and B having d<NUM> (median diameter), d<NUM> and d<NUM> shown in Table <NUM> were prepared.

As shown in <FIG>, a solid cylindrical SUS304 mandrel <NUM> of <NUM> in outer diameter for forming a lining layer <NUM> of a molding machine cylinder <NUM> was inserted into a cylindrical body <NUM> of <NUM> in outer diameter, <NUM> in inner diameter and <NUM> in length, which was made of microalloyed steel having a composition comprising <NUM>% by mass of C, <NUM>% by mass of Si, <NUM>% by mass of Mn, <NUM>% by mass of P, <NUM>% by mass of S, <NUM>% by mass of Cr, and <NUM>% by mass of V, the balance being Fe and inevitable impurities. A lid <NUM> made of the same material as that of the mandrel <NUM> was welded to one-side ends of the mandrel <NUM> and the cylindrical body <NUM>, to form an annular space <NUM> between the cylindrical body <NUM> and the mandrel <NUM>. The materials and shapes of the cylindrical body <NUM> and the mandrel <NUM> are shown in Table <NUM>.

The Ni-based alloy powder C and the tungsten carbide powder B were obtained at a mass ratio corresponding to <NUM>% by volume of the Ni-based alloy powder to <NUM>% by volume of the tungsten carbide powder, using the specific gravities of <NUM> for the Ni-based alloy and <NUM> for the tungsten carbide in the conversion of the volume ratio to the mass ratio. These powders were dry-mixed by a mixer, and the resultant powdery mixture 3a was charged into the space <NUM>. After the powdery mixture 3a was charged, a lid <NUM> having an evacuating aperture <NUM> was welded to the other ends of the cylindrical body <NUM> and the mandrel <NUM>. The space <NUM> containing the powdery mixture 3a was evacuated through the evacuating aperture <NUM>, and the evacuating aperture <NUM> was sealed by welding. The sealed cylinder containing the powdery mixture 3a was placed in a HIP apparatus to conduct HIP at <NUM> and <NUM> MPa for <NUM> hours in an Ar gas atmosphere. After the lids <NUM>, <NUM> and the mandrel <NUM> were successively removed from the HIPed composite cylinder by cutting, etc., the inner surface of the cylinder was finish-machined to obtain a molding machine cylinder (outer diameter: <NUM>, inner diameter: <NUM>, and length: <NUM>) having a <NUM>-mm-thick lining layer of a Ni-based alloy containing tungsten carbide particles on the inner surface of the cylindrical body. The types and amounts of the Ni-based alloy powder and the tungsten carbide powder used and the sizes of the machined cylinders for molding machines are shown in Table <NUM>.

A molding machine cylinder (outer diameter: <NUM>, inner diameter: <NUM>, and length: <NUM>) having a <NUM>-mm-thick lining layer of a Ni-based alloy containing tungsten carbide particles on an inner surface of a cylindrical body was produced in the same manner as in Example <NUM>, except for changing the types and amounts of the Ni-based alloy powder and the tungsten carbide powder as shown in Table <NUM>.

A molding machine cylinder (outer diameter: <NUM>, inner diameter: <NUM>, and length: <NUM>) having a <NUM>-mm-thick lining layer of a Ni-based alloy containing tungsten carbide particles on an inner surface of a cylindrical body was produced in the same manner as in Example <NUM>, except for changing the cylindrical body and the mandrel as shown in Table <NUM>, and the types and amounts of the Ni-based alloy powder and the tungsten carbide powder as shown in Table <NUM>.

A molding machine cylinder (outer diameter: <NUM>, inner diameter: <NUM>, and length: <NUM>) having a <NUM>-mm-thick lining layer of a Ni-based alloy containing tungsten carbide particles on an inner surface of a cylindrical body was produced in the same manner as in Example <NUM>, except for changing the cylindrical body and the mandrel as shown in Table <NUM>, the types and amounts of the Ni-based alloy powder and the tungsten carbide powder as shown in Table <NUM>, and further the HIP time to <NUM> hours.

A measurement sample was cut out of an end portion of the lining layer of the HIPed composite cylinder, to evaluate the maximum length of the matrix, and the mechanical strength, wear resistance, corrosion resistance and machinability of the lining layer in the molding machine cylinder, by methods described below.

Diagonal lines were drawn on a SEM photograph of each field (field area: <NUM> × <NUM>) of a cross section of the lining layer taken at magnification of about <NUM> times (acceleration voltage: <NUM> kV), and the lengths of line segments of each diagonal line crossing matrix regions composed of the Ni-based alloy <NUM> with no tungsten carbide particles <NUM> and no boundary phases <NUM> were measured. The maximum length in each field was selected, and the maximum lengths of the matrix in <NUM> fields were averaged (see <FIG>). The SEM photographs of cross sections of the linings of Examples <NUM> and <NUM>, and Comparative Examples <NUM> and <NUM> are shown in <FIG>. In the figures, black portions are the matrix regions composed of the Ni-based alloy <NUM>, white portions are the tungsten carbide particles <NUM>, and gray portions adjacent to the tungsten carbide particles <NUM> are the boundary phases <NUM>.

The mechanical strength was evaluated by bending strength measured by a four-point bending test of each cut-out measurement sample having <NUM>-C-chamfered longitudinal edges (thickness: <NUM>, width: <NUM>, and length: <NUM>), at an upper inter-fulcrum distance of <NUM>, a lower inter-fulcrum distance of <NUM>, and a test speed of <NUM>/min.

A round rod test piece of <NUM> in diameter and <NUM> in length was cut out of an end portion of the HIPed composite cylinder, such that the lining layer existed in one end surface. The end surface having he lining layer was finished by grinding. While rotating this test piece around its center axis at <NUM> rpm, the lining-layer-existing end surface was pressed to a #<NUM> grinder paper at a load of <NUM> N for <NUM> minutes two times, and the wear weight of the test piece was measured. The wear weight was divided by a test surface area to determine the amount of wear.

Test pieces (<NUM> × <NUM> × <NUM>) were cut out of an end portion of the lining layer of the HIPed composite cylinder. These test pieces were immersed in an <NUM>-% aqueous hydrochloric acid solution at <NUM> and a <NUM>-% aqueous nitric acid solution at <NUM> each for <NUM> hours, to measure their weight loss by corrosion.

Using a vertical honing machine (FVG-1500SA available from Fuji Honing Industrial Co. ), a honing head having four diamond grinders (<NUM> × <NUM> × <NUM>; and abrasive size: #<NUM>) circumferentially arranged at circumferential intervals of <NUM>° was rotated at <NUM> rpm, and inserted into the HIPed composite cylinder from which the mandrel was removed by machining, such that it came into surface contact with the inner surface of the composite cylinder at an expansion pressure of <NUM> MPa. The honing head reciprocated longitudinally <NUM> times over the entire length of the cylinder to measure the amount of grinding the inner surface.

It is clear from Table <NUM> that the molding machine cylinders of Examples <NUM> and <NUM> each having a lining layer comprising <NUM>-<NUM>% by volume of tungsten carbide particles having a median diameter d<NUM> of <NUM>-<NUM> and the matrix composed of the Ni-based alloy, the maximum length of the matrix in an arbitrary cross section being <NUM> or less, had excellent mechanical strength (bending strength), wear resistance and corrosion resistance, as well as excellent machinability.

Because the molding machine cylinder of Comparative Example <NUM> contained <NUM>% by volume of tungsten carbide particles having a median diameter of <NUM>, it exhibited extremely poorer mechanical strength (bending strength), wear resistance and machinability than those of Examples <NUM> and <NUM>.

Though the molding machine cylinder of Comparative Example <NUM> met the requirement of the present invention of containing <NUM>% by volume tungsten carbide particles having a median diameter of <NUM>, it failed to meet the requirement of the present invention in that the matrix had the maximum length of <NUM>, exhibiting poorer wear resistance than those of Examples <NUM> and <NUM>.

Claim 1:
A cylinder for a molding machine comprising a HIP-sintered lining layer on an inner surface of a cylindrical steel body; characterized in that
said lining layer (<NUM>) comprises <NUM>% by volume to <NUM>% by volume of tungsten carbide particles (<NUM>) having a median diameter d<NUM> of <NUM> to <NUM> and a matrix composed of an Ni-based alloy (<NUM>);
said lining layer (<NUM>) having a structure in which the tungsten carbide particles are uniformly dispersed in the matrix composed of the Ni-based alloy; and
the maximum length of said matrix in an arbitrary cross section is <NUM> or less, wherein the median diameter d<NUM> of the tungsten carbide particles dispersed in said lining layer (<NUM>) is that measured on tungsten carbide powder as a raw material by a Microtrac particle size distribution meter, and
wherein the maximum length of said matrix is determined by drawing two diagonal lines on a SEM photograph taken at a magnification of about <NUM> times, measuring the lengths of line segments of each diagonal line crossing matrix regions composed of the Ni-based alloy with no tungsten carbide particles and no boundary phases, and averaging the maximum lengths in <NUM> fields, wherein the SEM photograph has a field area is <NUM> × <NUM> and is obtained by using an acceleration voltage of <NUM> kV.