The Corrosion of Saw Blade Steel and Tipping Material
This article was taken from the below published research of College of Brunel University- Department of Timber and Construction. It has some really great information about the corrosion of cutting tools as a result of the acids in the types of wood being cut.
The Corrosion Of Sawblade Steels And Tipping Materials
G.D.Livingstone, G.J.Hall, J.Rose
Department of Timber & Construction
A College of Brunel University, UK
(Believed to be mid 1980’s)
The process of the corrosion of sawblade steel was studied in this pilot project with a range of four commercial timbers and three standard types of sawblade steel.
The effects of chemical corrosion produced a loss in weight of up to 2gm4h-1 and 0.0005mmh1 loss in thickness in the steels. The interaction between different timber species and compositions of steel was significant and could be commercially important.
In addition to the study of the corrosion of the sawblade steels three sawtooth tipping mechanisms were examined in a similar manner. Results showed that the hardness of a tip did not necessarily guarantee longer tooth edge life when the chemical aspect of wear was taken into account.
The wear of sawblade steel, and subsequent cracking, and the dulling of sawblade tips is a result of a set of complex interactions of metallurgical, physical and chemical influences.
In the past the emphasis has been placed on the mechanical aspect of damage, but the application of electrochemical principles has led to a greater understanding of the chemical effects. Studies by Kivimaa (1952), Alecseev (1957), Klamecki (1978) and Mohan & Klamecki (1982) have shown that the chemical factor in sawblade wear is of some importance. However, this has remained largely unquantified, mainly due to the complex chemistry of wood extractives such as organic acids and polyphenolic substances, some of which are capable of forming organo-metallic complexes. When activated by the appropriate electro-potentials these complexes are involved in the corrosion reactions (Hillis & McKenzie 1964).
A number of authors, namely Hillis & McKenzie (1964), Kirbach & Chow (1976) and Krilov (1985) have observed the simultaneous effects of chemical and physical wear on sawbiades operating under industrial conditions.
This paper shows the results of a pilot project, focusing on the corrosion aspect and not on abrasion.
The Corrosive Effects of Wood Extractives
The corrosion of cutting tools is due to the acidity of wood and to the chelating materials in the secondary components.
The acetyl groups present in the galactoglucomannans of softwoods and in the xylans of hardwoods are slowly and autocatalytically converted into acetic acid within the living tree.
The acidity of heartwood is therefore often high in older trees, especially in hardwoods. Wood in the centre of old, living eucalypt trees has been reported to have a pH value as low as 2.6, although values of pH 3-4 are more common. Storage of undried hardwood and softwood at temperatures up to 70°C and above also increases acidity (Feist, Hajny and Springer 1973).
A number of organic acids, namely Acetic, Formic, Oxalic, Gallic and Ellagic Acids are normally found in the extractives of most timbers, and occur in both the free and bound states. Ether simple organic salts or complex aromatic polyphenols, which are non-structural components of most plants, contain hydroxyl groups which are capable of forming soluble or non-soluble organo-metallic complexes. These acidic polyphenols, which are present in the tannic acid content of timber, have been found to play an important part in the corrosion of metals used in association with timber (Krilov 1986).
Acetic acid, from the hydrolysis of the acetyl groups of hemicelluloses has long been considered a main factor in the corrosion of metals in contact with wood. Acetic acid vapour is highly corrosive with as little as 0.5 parts per million being significant. Acid vapour can emanate from wood in conditions of high humidity and heat; conditions found in the proximity of sawblades used to convert green timber.
The effects of other volatile acids (e.g. Formic, oxalic and gallic) in wood extractives is much less known than acetic acid.
In experiments with some commercial Australian hardwoods, Krilov and Gref (1986) discovered that the polyphenolic content of the tannic acid element of extractives could be as high as 17% in both sapwood and heartwood. These aromatic phenols are capable of producing chelate with active ferric ions. Some of these pholyphenols also contain a carboxylic acid group, examples include gallic, digallic and ellagic acids.
During the formation of the chelate, the ferric ions are removed from the acid – metal equilibrium, which is then free to produce more ferric ions from the metal, and thus allowing the reaction to continue, and in some cases increase. For this reaction to occur, it is essential that the iron is initially oxidised to ferrous ions, and dissolved before it can combine with the appropriate polyphenol (Hillis and McKenzie 1964).
This suggests that the initial reaction in the corrosion of the steel would be the acidic attack on the iron content, which would result in the production of ferrous ions, which then, in the presence of air, would be oxidised to the ferrous state, with these ions being able to react with the polyphenols to produce chelates. Since the production of chelates would be limited by the number of ferric ions available, then the natural acidity of the wood would be of extreme importance in the production of active ions.
Therefore, within the general reaction scheme for the corrosion of sawblade steels, the production of ferric chelates is of high significance.
Although the general reaction scheme appears simple, in practice, because of the complex nature of wood extractives, the chemical corrosion of steel in timber is extremely complicated, and not yet fully documented. This is in part due to the differing complexes and content of polyphenols in the many species of timber, and even in the variability within species.
Another significant factor is that many species (pines, spruces, eucalypts) which are now being grown quickly, and therefore inevitably producing more juvenile (core) wood could also be producing larger proportions of polyphenolic compounds.
Under conditions conducive to electrochemical action such as in wet wood, the major part of the total wear of cutting is due to corrosion (Mohan and Klamecki 1982).
The extent of the electrochemical action has been shown to be influenced by the amount and type of tool binder material. Nickel being a more suitable binder for cemented tungsten carbide than cobalt. Iron being an unsuitable binder (Mohan and Klamecki 1982).
While acidity is a contributing cause, the presence of compounds capable of chelating with metals is probably a greater contributor to the extent of corrosion. For example, Tropolones, which chelate with a range of metals (Gardner 1962) are present in incense cedar (Calocedrus decurrens – Libocedrus decurrens) but not in ponderosa pine (Pinus ponderosa) leading to more rapid wear of high speed tools by incense cedar. The tropolones in western red cedar (Thuja plicata) preferentially remove cobalt from cemented carbide tools (Kirbach and Chow 1976).
The Dulling of Sawtips by Chemical Action
The dulling of sawteeth is a critical factor in the commercial viability of the saw. The sharpness of the cutting edge profoundly affects the surface finish of the timber, the accuracy of the cut, and as a result, the total yield from the log. Part of the effort to increase yield has resulted in more attention being paid to reducing the wear on the cutting edge, particularly in thin kerf sawing, where the performance of the saw is seriously affected by the condition of the cutting edge.
In sawmills wood is generally cut wet, and therefore chemical wear of the cutting edge can seriously reduce the yield of the mill. Chemical wear can be more pronounced in certain species. Kirbach (1982) produced evidence that the chemical wear in the cutting of unseasoned western red cedar (Thuja plicata) accounted for 70% of the overall dulling of the cutting edge, whereas in unseasoned spruce (Picea spp.) the chemical element of dulling amounted to only 30% of the overall wear. Krilov (1986) observed that steels of slightly differing composition might react differently to the chemical wear from a certain species of timber and suggested that for a certain species of timber there might be a steel of specific composition which would corrode less than others and generally provide a better performance.
The two most favoured methods of tipping sawblades, tungsten carbide and Stellite (R), both suffer from dulling as a result of abrasion and chemical wear, although the latter has not been identified on an individual basis. Polycrystalline diamond remains to be investigated. In tests carried out by Kirbach and Bonac (1981) Stellite (R) 12 was found to perform better in cutting tests than tungsten carbide, despite the fact that in earlier studies the dulling of both tipping systems was similar. This difference in results was attributed to the interaction between the chemical and mechanical components of dulling which appeared to be more pronounced in the tungsten carbide. This was reiterated by Kirbach (1982) when he stated that despite the fact that Stellite (R) 12 has a hardness of 47-51HRC, whilst tungsten carbide has a greater hardness of 82HRC, Stellite (R) 12 still out-performed tungsten carbide by a ratio of 2:1. This surprising observation is based on the high corrosion resistance of Stellite (R) 12 because of its higher chromium content (29%).
Tungsten carbide is more efficient in cutting dry wood however and there is very little difference in the performance of Stellite (R) 12 and tungsten carbide in cutting seasoned timber. This is because there is only one wear component in sawing dry timber, that of abrasion, whereas in the cutting of wet timber the corrosion aspect comes into consideration.
It appears that the chemical component attacks the cobalt matrix of the tungsten carbide at the interface, leaving the tungsten particles protruding from the tip and subject to greater abrasive forces than normal in the cutting action, thus allowing them to be more easily removed by abrasion. This action accelerates the blunting of the tip. In the case of Stellite (R) tips the same chemical and abrasive wear takes place but at a smoother and slower rate than with the tungsten carbide. Kirbach (1982) declared that the superior performance of the Stellite (R) 12 was as a result of its greater resistance to wood acids.
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