Harden silverstaal

[Didado]

freaky wrote:

Hallo allemaal !!

Wie kan mij vertellen hoe ik Silverstaal kan harden ??

Ik hoorde dat je het kersenpitten rood moet stoken, en dan moet afkoelen in olie.

Het ontlaten van het mat. is mij niet helemaal duidelijk.

Kan iemand mij ook vertellen hoe warm het mat. bij beide behandelingen moet zijn?

Groetjes Erik.

15.1 Silver Steel (Drill rod)

Silver steel is a commercial low-carbon tool steel produced as accurately ground lengths of round and square bar. Common lengths are 13″ and 18″ (though any reasonable length can be had from the factory), and diameters up to about 1″. It is sold in the semi-soft or ‘annealed’ state. The steel comprises 1.1/1.2% carbon, together with the addition of about 0.35% manganese, 0.45% chromium and 0.1-0.25% silicon. It is particularly useful in the home workshop because it can be easily machined prior to hardening, the latter process being quite simple to carry out with nothing more than a small propane torch and a bucket of water.

Lets take for example a simple 1/2″ diameter D-bit reamer, to be made from silver steel and hardened & tempered. The basic shape of the tool would be produced whilst the steel is still in the soft state, being machined or filed to exact dimension. Round silver steel is ground to close tolerance (-0.0000″ to +0.0005″ on nominal size) so a piece of 1/2″ dia rod can be used and simply be cut to length. The metal surfaces should be polished either with fine emery or on a buffing wheel to remove scratches which may later form weak points leading to fracture (not too important for our D-bit but it would be for a home-made tap). The work is then heated to ‘cherry’ red (somewhat above the transition temperature of 800 degrees) to form the iron carbide before quenching in cold clean water. Brine is actually a little better as the bubbles formed are finer. Cherry red is a term often used but in fact is quite difficult estimate, being dependent as it is upon the levels of ambient light. Perhaps a better method is to use the disappearance of magnetic attraction as a measure of the critical temperature. Quenching should be done by dipping the hot metal bar vertically into the water then swirling it around a little, this technique will minimise any tendency to warp which might occur should one side cool (and therefore shrink) appreciably faster than the other.

Upon removal of the D-bit from the quench it will be noted that the surface is black and probably scaled. An old-time tip to avoid this is to coat the metal with soft soap prior to heating, upon quenching the metal will retain a nice even silvery-grey surface. In any case, the metal now needs to be polished back to base metal to facilitate the tempering process. One characteristic of polished steel is that upon heating through a range 150 to just above 300 degrees C it progressively changes colour from faint yellow through straw, brown, blue and finally near black. We can use this colour change to indicate the temperature of the metal and so control the drawing of the temper. Each stage of the colour change indicates a percentage of the carbide compound remaining in the steel, and therefore it’s hardness. The objective of tempering is to reduce the hardness to the point required and then stop the carbide breaking down any further by immediately quenching in cold water. Useful indicators are:

Pale yellow – hardness 750-800 Vickers – Still quite brittle but very hard.

Pale straw – hardness 720-770 Vickers – Suitable for machining mild steels. Low resistance to fracture.

Dark straw – hardness 700-750 Vickers – Suitable for home made taps, scribers and the like.

Purple-blue – hardness 650-700 Vickers – Suitable for chisels and punches.

Dark blue – hardness 640-690 Vickers – suitable for springs.

You can see from the above that there is considerable range in the actual hardness achieved, and this is due variations in temperature (for which colour is only a relatively crude indicator) and also variations in the carbon content of the parent steel. Nevertheless, it’s clear that a home-made tap tempered to pale-yellow might be useful for tapping hard materials but would be very likely to snap in use due to it’s low tensile strength. Our D-bit is unlikely to be subject to the same tensile stresses the tap would be so we can get away with somewhere between pale-yellow and pale-straw to take advantage of the extra hardness. Note that these temperature are relatively low and, for a lathe toolbit, or home made milling cutter destined for arduous work, such temperatures might be achieved in use. It is clear then that hardened low carbon steels must be kept cool if they are to retain their hardness, a property which is the main reason why ‘high-speed’ steels have displaced them for normal machine tool usage.

The appearance of 5 pieces silver steel hardened and tempered as described in the table above.

The D-bit example is fairly straight-forward and is a good place to try out your metal hardening skills. However, there are many jobs that require differential tempering such that one part of the tool is softened and another stays hard. It is here that some skill comes in to play. Imagine a simple metal rod some 1/4″ diameter, it’s possible to heat this rod with a small flame some 2-3″ from the end, and watched carefully the colours will be seen to ‘run’ towards the end. It is quite possible at this point to remove the source of heat and watch the play of colours and just when the correct colour is achieved at the tip quickly quench. This will give a range of hardness that ranges from perhaps very hard at the tip to soft some 2 inches further along. You can control this process even more using ‘heat sinks’ which will slow the rise in temperature at one point but not at another. A favourite trick is to sink the end which needs to remain hard in a potatoe which will then keep that portion relatively cool. Other workpieces may be of such fine structure that it is difficult to apply the heat evenly. In this a case it is necessary to place the object in a sand tray and heat the lot from below, a larger piece of steel is usually included in the sand bath to act as a visible indicator of the temperature attained, and once attained the whole lot is dropped into the quench bucket. This method is particulaly used for small metal parts (like watch hands) to produce a nice even blue colour, and also for springs to prevent the formation of hard spots which might lead to subsequent fractures.

David

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