HARDENED HEAT TREATMENT CONTROL

Obtaining an inspection surface requires a succession of operations, each as important as the next, regardless of the material treated. These steps are in the following order:
• The removal of the product to be examined (if necessary), called “CUTTING”.
• Standardisation of the geometry of the sample taken (if necessary), called “MOUNTING”.
• Improvement of the surface condition of this sample, called “POLISHING”.
• Characterisation of the sample: revealing the microstructure of the sample by an etching reagent (if necessary) called “METALLOGRAPHIC ETCHING” and microscopic observation (optical or electronic).
=> Each of these steps must be carried out rigorously, otherwise the following steps will not be possible.

The purpose of cutting is to remove a precise section of a product, in order to obtain a suitable surface for inspection, without altering the physico-chemical properties of the steels in question.
In other words, it is essential to avoid heating or any deformation of the metal that could lead to degradation of the material. Cutting is fundamental step which conditions the further preparation and inspection of parts.

PRESI’s wide range of medium and large capacity cutting and micro-cutting machines can be adapted to any need with regard to cutting precision, sizing or quantity of products to be cut:

All cutting machines are used with a lubricating/cooling liquid composed of a mixture of water and anti-rust additive in order to obtain a clean cut without overheating. The additive also protects the sample and the machine from corrosion.

Table 1: Choosing the right cut-off wheel type

=> The choice of the cut-off wheel type has to be adequate, in order to avoid cutting failure, or excessive cut-off wheel wear or even breakage. The hardness of the workpiece determines the wheel selection.

Samples can be difficult to handle due to their complex shape, fragility or small size. Mounting makes them easier to handle by standardising their geometry and dimensions.
=> Achieving good-quality mounting is essential to protect fragile materials and also to achieve good preparation results for polishing and future analysis.
Before mounting, the sample should be deburred with coarse abrasive paper, for example, to remove any cutting burrs. Cleaning with ethanol (in an ultrasonic tank for even greater efficiency) is also possible. This allows the resin to adhere as well as possible to the sample and thus limits shrinkage (space between the resin and the sample).
If shrinkage persists, it can lead to problems during polishing. Abrasive grains may become lodged in this space and then be released at a later stage, thus creating a risk of pollution for the sample and the polishing surface. In this case, cleaning with an ultrasonic cleaner between each step is recommended.

 

There are two mounting options:
• HOT MOUNTING is to be preferred for edge inspection purposes or if the metallographic preparation is carried out in preparation for hardness testing. This option requires a hot-mounting machine.

COLD MOUNTING is to be preferred:
• If the parts to be examined are fragile/sensitive to pressure
• If they have a complex geometry such as a honeycomb structure.
• If a large number of parts are to be mounted in series.

Fig 13: Pressurized mounting device
The cold process can be used with:

Cold resins do not always provide a flat mounting “back” because of the meniscus of the liquid resin. Before any polishing operation, a brief step  using abrasive paper will remove this meniscus. The important thing is to ensure that this operation renders the two sides of the mounting parallel.

To meet user needs, PRESI offers a full range of cold mounting moulds. The cold process has different mounting moulds with diameters from 20 50mm. These are divided into several types: optimised moulds called “KM2.0”, rubber, Teflon or polyethylene moulds.

Cold mounting is also more flexible, hence the existence of rectangular moulds for more specific needs.

The last and crucial phase in the sample preparation process is polishing. The principle is simple, each step uses a finer abrasive than the previous one. The aim is to obtain a flat surface and to eliminate scratches and residual defects that would hinder the performance of metallographic control examinations such as microscopic analysis, hardness tests, microstructure or dimensional inspections.

PRESI offers a wide range of manual and automatic polishing machines, with a wide choice of accessories, to cover all needs, from pre polishing to super-finishing and polishing of single or series samples.
For hardness testing, polishing with an automatic polishing machine from the Mecatech range seems to be the most suitable.

All the polishing ranges below are given for automatic sample preparation (for manual polishing: do not take into account the parameters at the top). They are the most commonly used and are given for information and advice. All the first steps of each range are called “levelling” and consist of removing material quickly to level the surface of the sample (and resin). Those given below are standard and can therefore be modified as required.
Applied pressures vary according to sample size, but in general the following applies: 1daN per 10mm mounting diameter for the pre-polishing steps (ex: Ø40mm = 4 daN) then reduce force by 0.5daN at each polishing step with an abrasive suspension.

This range is suggested for superficially and core-treated steels:

NB: Levelling with the I-Max R 54μm is sufficient for a sample issued from a metallographic cut. If more material has to be removed, an I-Max R with a larger grain size (75μm or even 125μm) should be used.

Grinding is performed using I-Max R discs. These resin-bonded diamond discs can replace several hundred abrasive papers. They provide good flatness for polishing hard ferrous materials.
When grinding, it is not necessary to reverse the rotation direction of the head and platen as this can detrimentally affect flatness. However, reversing the directions of rotation can help if a large amount of material has to be removed.
A 3-step range is sufficient for hardness testing. The scratch pattern obtained with the 3μm suspension is fine enough to allow the hardness indentations to be read. The RAM cloth can also be replaced by ADRII cloth.

If structural observation is required, a 1μm finish on an NT cloth can follow.
Finally, a superfinish with PRESI n°3 alumina can be used to make an inclusionary check for example.

This is a second possible range for superficially or core-treated steels:

For this second range, step 2 is replaced by an MED-R disc. This disc, composed of resin pads, makes it possible to replace several cloths and maintain good flatness. It is used with a super abrasive suspension for MED-R which is a 2-in-1 product (the abrasive suspension and lubricant are already mixed and ready to use).
For diamond polishing, steps 3 and 4 are carried out using LDP concentrated polycrystalline suspensions. Polycrystalline diamond has sharp angles which are suitable for polishing medium-hard to hard materials.
If the materials to be polished are sensitive to corrosion, LDP diamond suspensions can be replaced by alcohol-based, anhydrous polycrystalline ADS diamond suspensions.

 

Sometimes it is necessary to adapt the range according to the treatment, especially in the case of
nitrided steels.

NB: Levelling with P320 abrasive paper is sufficient for a sample issued from a metallographic cut. If more material needs to be removed, a larger grit size abrasive paper should be used.

For nitrided steels, polishing with I-Max R or MED-R is too aggressive and could damage the combination layer. It is therefore preferable to replace the I-Max R with a P320 abrasive paper.

The following steps are carried out conventionally using polishing cloths and LDP diamond suspen- sions and the associated lubricant Reflex LUB. Finally, superfinishing is optional using alumina on an NT flocked cloth.

Treated steels are generally etched using Nital 4% etching solution (except for stainless steels which have their own etching solution). This reveals the structure and, in the case of nitrided steels, highlights the combination layer (white layer). All of the micrographs presented were produced using the PRESI VIEW software:

Usually, hardness tests are carried out to check the conformity or otherwise of the heat-treated part. This will therefore show variation in hardness if the treatment is superficial (harder externally and less hard throughout) and a lesser variation (with fairly similar hardness values) throughout the workpiece if it has been treated in the mass.

To carry out these hardness tests, the HZ 10-4 hardness tester should be used, with the PRESI Touch Pattern software.

Fig. 28: HZ 10-4 hardness tester

Fig. 31: Graph of induction-treated steel