INOX – Steel processing guide
6- HOW TO WORK THEM
Working with stainless steels requires knowledge of some of their special characteristics and needs taking into account the attitudes of the different alloys to machining and structural changes that can occur during the processing stages.
First of all, you should understand the differences with the carbon steel: picture 1 shows the approximate tensile-elongation diagrams for austenitic stainless steel (AISI 304), for martensitic and ferritic stainless steels (AISI 410 and AISI 430) and for a generic common steel.
The diagram shows that martensitic and ferritic grades have the same type of curve, in which we observe a trend similar to the carbon steel trend, with a well identifiable yield point. Austenitic steels, instead, have a completely different behavior: while there is not a well defined yield point and therefore they do not allow a real collapse limit.
Consequently, for these steels, we identify a conventional value of yield strength, adopting that of the stress that causes a permanent deformation of 0.2%. In addition, breaking loads can be found but, above all, also elongations to break much higher than the ferritic and martensitic stainless steels and also than carbon steels.
This means that the steel, particularly austenitic ones, have a strong attitude to be cold formed; however, undergoing the hardening phenomenon, that is the raising of the mechanical properties: tensile strength, yield strength and surface hardness. Therefore in the course of the working, the steel, especially austenitic, varies its characteristic of strength; It will be necessary to use different tricks than those followed for the processing of conventional steels.
Let’s do a quick rundown on the precautions to be followed for the most important types of work.
The increase of the yield strength causes, in processes for cold plastic deformation, an elastic return (expecially for austenitic steels), greater than that found in carbon steel: 2 to 3 times greater for the steel of the 300 series. So it is indispensable, in bending operations, predict an appropriate angle of over-bending so that, at the “release”, you get the desired bend value.
In picture 2 it is schematically shown the qualitative shape of the different elastic returns detectable in bending at 90° of carbon steel plates, stainless steel (300 series) solubilized and of the same steel, strongly hardened.
In cold plastic deformations, as can be seen from table 2 (point 3), the elongation at break is very high, especially for the austenitic series. As a result, it is possible to impose higher reduction ratios than with standard steel.
An austenitic steel, such as AISI 304, allows to reach without difficulty reduction ratios in the order of 40% and over, with a single operation. In special cases, it can even reach values of 50÷55%; however, it will be necessary to eliminate internal stress in a short period of time (a few hours), in order to avoid that they cause cracks in the manufactured piece.
A martensitic stainless steel, such as AISI 410, allows to obtain reductions of about 25% and, in any case, not higher than 30% in a single operation, without having to resort to intermediate annealing.
A ferritic stainless steel, such as the AISI 430 type, can have a deformation of about 25÷35%, in one operation, without intermediate annealing. This steel is less deformable than austenitic one and, for this reason, in order to obtain a better flow of the material under the blank holder, using lower pressures, thus avoiding excessive thinning of the strained areas.
The load of a high yield that is reached during the plastic deformation, usually requires a greater load on the punch before arriving at the plastic state. The hardness, also higher, requires the use of harsher equipment as those used for the carbon steel. Both the matrix and the punch must in fact be capable of supporting greater pressures and to offer a better wear resistance.
In table 3 are gathered some indicative data referring to the characteristic geometric parameters of the press-drawing, according to the thickness of the deep-drawn sheet metal, for three stainless steels and for the carbon steel for deep molding.
Table 3 – Experimental values of curvature radius of the edge of the matrix, of the bottom of the punch and the punch-matrix radial clearance for three different types of stainless steels, depending on the sheet thickness. Values for carbon steels for deep molding are also reported.
In the case of mechanical cutting (shearing with scissors, cutting, etc.), you must pay particular attention to the clearance between the blades. In fact, due to the considerable elongation at break, as described above, the material, very deformable, can penetrate into the free space left between the blades and, if the clearance is too large, it gives rise to the formation of very hard burrs given the considerable work hardening meted out to the material. Such burrs facilitate the wear of the cutting devices, increasing the clearance and hindering the processing itself.
Usually, it is good to maintain a clearance of at least 1/10 of the sheet thickness to be cut (see Fig. 3).
For shearing (for example that of the discs), it is necessary to use a speed lower than that which would be used in a similar manner for carbon steel plate (approximately 2/3), while the power of the machine must be increased by approximately 50%.
PROCESSES WITH MATERIAL REMOVAL
Stainless steels can be regularly machined with machine tools, in all the presented types: austenitic, ferritic and martensitic. For these processes, however, it is good to keep in mind certain precautions:
- The machine must be rigid and must have a sufficient power margin. It is appropriate to use the machine not more than 70÷80% of the available power.
- Tools must be very hard, together with their supports, so as to avoid as much as possible the vibrations and take full advantage of the machine stiffness qualities.
- Tool materials should be appropriately chosen from high-speed steel, metal carbides, ceramics. It is also necessary, as far as possible, that dimensions of tools are large, to allow greater stiffness and a greater dissipation of heat in the cutting zone. This is because the steel, in particular austenitic, have a low thermal conductivity.
- The geometry of the tool must be chosen with the most appropriate characteristic angles; sharpening must also be very accurate and restored frequently.
- It is appropriate to use abundant cutting fluid, addressed in the work area, to allow maximum heat dispersion.
- The choice of cutting parameters is important: in particular, the depth of cut must be such as to ensure the cut below the hardened area from the previous pass.
To facilitate the removal of chippings, are available stainless steels with improved machinability, containing suitable quantities of sulfur or selenium.
These elements, dispersed in the matrix, allow a smoother chip fragmentation and develop lubricant action, thus increasing the workability.
The most common stainless steels with improved machinability are the following:
- Austenitic: AISI 303; If AISI 303; AISI 316 F
- Ferritic: AISI 430 F
- Martensitic: AISI 416
whose analyzes are shown in Chapter 3.