Refining element for a refining disc

Abstract

Refining elements for refining discs for disc refiners for the manufacture of mechanical pulp, for example, board pulp, are subjected to hard abrasive wear in acid environment and at high temperature. In order to improve the wear resistance alloys with precipitated carbides are used. A service life improved in comparison with known alloys is obtained with a refining disc with the following analysis in % by weight: 2.96 C, 0.77 Si, 0.82 Mn, 24.2 Cr, 5.16 V, 0.04 Ni, 0.03 Mo, and the remainder Fe and impurities. After casting, the refining segments are hardened and annealed and assume a hardness of 57-63 HRC.

Description

TECHNICAL FIELD

[0001] This invention relates to refining elements for a refining disc for disc-refiners intended for the manufacture and/or treatment of fibrous pulps, where the refining element is produced by casting a steel alloy and hardened and heat treated to a hardness of at least 55 HRC.

BACKGROUND OF THE INVENTION AND KNOWN STATE OF ART

[0002] Disc-refiners for the refining of lignocellulosic material, i.e. for the mechanical manufacture or treatment of so-called mechanical pulp, are known, for example, through SE 506 822 and 402 019. They comprise two circular refining discs, which are rotated relative to each other, and have refining surfaces built-up of refining elements (normally called refining segments), which comprise bars and grooves and guide the pulp from the centre out to the periphery during the refining operation. The refining surfaces are subjected to heavy abrasive wear due to foreign hard particles, sand, in the chips. The temperature, besides, is high, often about 220° C., and the wood yields an acid pulp with a pH, which at the making of newsprint is about 6.5, but at board manufacture is as low as 4-5, which requires corrosion resistance. In order to reduce the wear, alloys with precipitated carbides are used.

OBJECT OF THE INVENTION AND DESCRIPTION OF THE INVENTION

[0003] The invention has the object to provide refining elements of the aforedescribed kind, which have an improved service life. This object is achieved in principle in that the refining disc has the following analysis, in per cent by weight: 2.7-3.2 C, 0.5-1.0 Si, 0.7-1.2 Mn, 21.0-26.0 Cr, 3.0-6.0 V. at maximum 0.5 Ni, at maximum 0.5 Mo, and the remainder Fe and impurities.

[0004] Vanadium is a very strong carbide former with a considerably greater affinity to carbon than chromium and the vanadium carbide has a hardness, which clearly exceeds that of the chromium carbide. Already at the solidification a precipitation of vanadium carbides is obtained which improves both the wear resistance at abrasive wear and the corrosion resistance. The last-mentioned property is understood and explained in that every per cent of vanadium binds up to 0.23% carbon. As a result thereof the carbon content in the matrix decreases in corresponding degree, which has the consequence that the carbon content available for chromium for the formation of carbides becomes lower. The chromium carbides, which are precipitated, contain also a certain amount of vanadium. Therefore, the proportion of chromium substitution-solved in the matrix, which improves the corrosion resistance, increases. Due to the adapted contents of carbon, chromium and vanadium the primary precipitated carbides assume a desired size, so that the tenacity is not reduced thereby that the primary precipitated carbides are too great. The fracture surfaces of the alloys according to the invention are considerably more fine-grained than of the other chromium-alloyed casting alloys for refining elements. A material, thus, is obtained which has both improved resistance to abrasive wear and improved corrosion resistance. This is important especially for refining segments, which are intended to be used for the refining of board pulp.

[0005] In order to achieve sufficient hardness of above 55 HRC, usually 57-63 HRC, after hardening and heat treatment, the hardenability must be sufficient. Therefore, the carbon content must be kept high, and if the stated analysis interval for the carbide forming elements Cr, Mo and V and for Ni are exceeded, the hardness will not be achieved. If the carbon content exceeds the stated analysis interval, the carbides grow and embrittle the material.

[0006] If the lower limits for Cr and V are fallen short of, the desired mixture of carbide types essential for the wear resistance is not obtained.

[0007] In order to achieve maximum wear resistance and tenacity, the material must be hardened and annealed in a conventional way. In connection with this heat treatment a secondary carbide fraction is precipitated, which is more finely dispersed than the one obtained at the solidification.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0008] An alloy according to the invention has been cast and heat treated and compared with two known alloys used for refining segments. The anlyses are shown in the following table, which also shows the results of the wear tests.

Table of chemical composition in percent by
weight and results of wear tests
								Wear
Alloy	C	Si	Mn	Cr	V	Mo	Ni	(mg)
Alloy acc.	2.96	0.77	0.82	24.2	5.16	0.04	0.03	170
to invention
Comparison	1.12	0.89	0.93	16.70	0.19	0.61	0.13	260
alloy 1
Comparison	2.63	0.43	0.75	26.60	0.35	0.02	0.10	200
alloy 2

[0009] In order to be able to evaluate and rank the properties of the alloys, an abrasive wear test is used where a definite sample size of the metal samples is worn against grinding paper for a definite period and with a constant pressure. The tests were made in water of room temperature. The abrasive wear resistance was measured as weight loss in milligram. Of each alloy three sample pieces were made, and four tests of each sample piece were carried out. The mean values are shown in the Table above. The lower the weight loss, the greater, thus, is the resistance to abrasive wear. The result cannot be directly converted into expected service life, because several parameters such as pH, temperature, rotation speed of the refiner, a.o. have an effect under the operation conditions. The test in laboratory environment has the advantage that one is not subjected to all imaginary variations arising at use in operation. As a completion full-scale tests in operation were carried out with refining segments of the alloy according to the invention, and a similar test with refining segments of comparison alloy 1. The refining segments had exactly the same surface pattern. The alloy according to the invention had a service life which was 80% better than that of the comparison alloy.

Claims

1. A refining element for a refining disc for disc refiners for the manufacture and/or treatment of fibrous pulps, where the refining element is produced by casting a steel alloy and hardened and heat treated to a hardness of at least 55 HRC, characterized in that the refining element has the following analysis in % by weight: 2.7-3.2 C, 0.5-1.0 Si, 0.7-1.2 Mn, 21.0-26.0 Cr, 3.0-6.0 V, maximum 0.5 Ni, maximum 0.5 Mo, and the remainder Fe and impurities.

2. A refining element as defined in claim 1, characterized in that the content of C is 2.8-3.1% by weight.

3. A refining element as defined in any one of the preceding claims, characterized in that the content of Si is 0.7-1.0% by weight.

4. A refining element as defined in any one of the preceding claims, characterized in that the content of Cr is 22.0-25.0% by weight.

5. A refining element as defined in claim 4, characterized in that the content of Cr is 23.0-24.5% by weight.