Patent Description:
Steels can be characterized by the presence of several crystalline phases (such as Ferrite, Martensite and Austenite), the formation and stability of which is regulated by various factors including the cooling rate, the chemical composition and the type of processing. Austenite, in particular, is a metastable phase that forms at high temperatures; in the cooling process it transforms into a more stable crystalline phase (such as Ferrite). In industrial production processes, under certain conditions, this transformation can occur incompletely. The residual austenite is the fraction of the austenitic phase that has not transformed. For high-performance steels, the accurate measurement of the amount of Austenite is a key factor.

The determination of the amount of residual Austenite is one of the fundamental controls in the production processes of steel components, as it contributes to define their mechanical performance. Checking the content of residual Austenite allows the production of specific components for different applications.

X-ray diffraction is one of the most used techniques to carry out this check in that the analysis is non-destructive. It is therefore of considerable interest to be able to have reference samples capable of validating the results obtained with the tools based on this technique.

The first reference samples (SRM) for the determination of residual Austenite by X-ray diffraction were produced beginning in <NUM> by the National Institute of Standards and Technology (NIST) by sintering austenitic and ferritic/martensitic steel powders. In parallel to the production, NIST has published procedures for the preparation and verification of SRM samples which describe the characteristics of the starting materials (particle size between <NUM>-<NUM> for ferritic powder, between <NUM>-<NUM> for austenitic powder) and the techniques used for characterization (X-ray Fluorescence and Optical.

Microscopy) are described [<NPL>; <NPL>] (also published as <NPL>)).

The production process described in the <NUM> standards is extremely complex as well as unreliable. Consider, for example, that <NUM> steps are proposed for the preparation of samples.

The procedure for determining residual Austenite by X-ray diffraction is described by ASTM E975 [ASTM E975-<NUM>, Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation, ASTM International, West Conshohocken, PA, <NUM>].

The applicability of the legislation, the first version of which dates back to <NUM>, was assessed by means of a Round-Robin test procedure in <NUM> [<NPL>. From this study it was found that, on heat treated steel samples <NUM>, the intra-laboratory repeatability is <NUM>% and the interlaboratory reproducibility is <NUM>% with a confidence interval of <NUM>%. In the same study, the reference samples produced by NIST were also analyzed for which discrepancies between the expected values and the results obtained by applying the ASTM E975 standard were found. The authors attribute these differences to the compositional characteristics of the SRM samples (content of alloying elements greater than <NUM>%).

The conclusions of this study were included in the current ASTM legislation, in particular in paragraph <NUM>, which deals with the applicability of the legislation, and in paragraphs <NUM> and <NUM>, which concern the accuracy of the results that can be obtained. Here it is also specified that it is not possible to determine the deviation of the values obtained from the real ones since currently there is no independent method to determine the content of Austenite in a reference sample.

The production and sale of NIST samples was definitively discontinued starting from <NUM>, probably also as a result of the study carried out by the American research laboratory Lambda Technologies. In a <NUM> publication [<NPL>] Lambda Technologies itself mentions that the quality of the samples was not sufficient to ensure adequate accuracy in quality control processes.

The measurements carried out by the Applicant on reference samples currently available on the market have shown a behavior similar to that found in <NUM>, i.e. important discrepancies (<NUM>-<NUM>%) between the results obtained by analyzing reference samples and real samples certified through the Round procedure Robin (interlaboratory).

The consistency between the results obtained on reference samples and on real samples is clearly an essential requirement for controlling the quality of a production and / or the development of new materials.

Starting from the information available on the NIST samples and from the data collected on the samples present on the market, the Applicant not only determined what are the necessary and sufficient conditions to create reference samples equivalent to real samples, but it did so using a minimum number of production steps, appropriately selecting the best grain size. The objectives of the present invention are:.

It is an object of the present invention to overcome the drawbacks of the prior art. In particular, it is an object of the present invention to define the method (laboratory materials and instrumentation) and the procedure for making homogeneous samples that can be used as a reference for the determination of residual Austenite for overcoming what is stated in Section <NUM> of ASTM E975-<NUM>.

A further object of the present invention is to have reference samples for the determination of residual Austenite by diffraction of rays that can be produced and marketed for the steel industry.

These and further objects of the present invention are achieved by means of a system incorporating the characteristics of the attached claims, which form an integral part of the present description.

The samples, object of the present invention, are prepared starting from a "quantity of the austenitic phase known a priori" or measured with independent instrumentation and more accurate and precise than that with which the sample will then be analyzed (for example a diffractometer).

"Austenitic phase quantity known a priori" means that the samples must have a precise quantity that can be obtained through "independent instrumentation" i.e., the Austenite content in the final sample is determined by a measuring instrument as and not by an inter-laboratory round-robin test.

The task of the present invention is concerned with a method for the preparation of reference samples for determining residual Austenite by X-ray as set forth in claim <NUM>.

As regards the starting materials, these must have a certified chemical composition (for both austenitic and ferritic steel powders) and an average particle size between <NUM> and <NUM>.

The characterization of the phase purity of the starting materials is carried out by X-ray diffraction. In a preferred aspect of the invention, the analyzes are carried out with diffractometers controlled with reference standards NIST SRM 640e and 1976b.

The particle size of the starting powders and the dimensional homogeneity of the two phases surprisingly allow for a linear and direct response between the theoretical content of Austenite (weighted quantity) and the content observed by X-ray diffraction.

The expression "dimensional homogeneity of the two phases" means that the samples are prepared starting from powders with different crystalline phases. The mixture is homogeneous when the two phases are homogeneously dispersed in the sample. The evaluation is done measuring the Austenite content on the two surfaces of the sample by X-ray diffraction, according to the ASTM E975-<NUM> standard. Three measurements are made on both surfaces; between one measurement and another the sample is rotated on itself randomly. This allows to evaluate the homogeneity of the sample.

Further characteristics and objects of the present invention will become more apparent from the following description.

The invention will be described below with reference to some examples, provided by way of non-limiting examples, and illustrated in the attached figures.

These figures illustrate different aspects and results of the present invention and, where appropriate, relevant reference numerals.

While the invention is susceptible to various modifications and alternatives, some preferred embodiments are shown in the examples and will be described below in detail. It is to be understood, however, that there is no intention of limiting the invention to the specific embodiment illustrated, but, on the contrary, the invention intends to cover all modifications, and equivalents that fall within the scope of the invention as defined in the claims. The use of "for example", "etc.", "or" indicates non-exclusive alternatives without limitation unless otherwise indicated. The use of "include" means "includes, but not limited to" unless otherwise indicated.

The reference samples for determining residual Austenite by X-ray diffraction are prepared starting from ferritic and austenitic phase powders, mixed in known quantities and pressed into a pellet with the addition of a binder.

The Austenite content in the final sample is therefore known a priori because it is measured with an independent instrumentation (analytical balance). The powders used for the preparation of the samples are of known and certified composition. The crystalline structure of the starting materials is also checked by means of a powder diffractometer before preparing the samples.

The starting materials are Ferrite and austenitic steel powders. Powders of all austenitic steels, for example AISI <NUM><NUM> or AISI <NUM>, and ferritic / martensitic, for example AISI <NUM> can be used. The steel powder must have a purity greater than <NUM>%. In a preferred form of the invention, <NUM> austenitic steel powder is used, having composition Fe / Cr18 / Ni10 / Mo3, and purity <NUM>%.

As far as the characteristic of the iron powder is concerned, it must have a purity greater than <NUM>%, preferably greater than or equal to <NUM>%.

As regards steel powder, the average particle size must be between <NUM> and <NUM>, preferably between <NUM> and <NUM>. In a particularly preferred aspect of the invention, the average nominal size of the steel particles is <NUM>. As for the iron powder, the average particle size must be between <NUM> and <NUM>, preferably between <NUM> and <NUM>. The mean size of the particles or granulometry is the value of the distribution dimensional corresponding to <NUM>% of the dust particles. The powders are certified for their chemical composition. It is particularly preferred that the average particle sizes of iron and austenite are similar.

With reference to <FIG>, the first phase of procedure <NUM> concerns the characterization of the phase purity of the powders by X-ray diffraction. The analyzes are carried out with diffractometers controlled with NIST SRM 640e and SRM 1976b reference standards.

"Characterization of the phase purity of the powders" means that the powders used for the preparation of the samples have a chemical composition certified by the manufacturer. As for the crystalline phases present, the materials are not certified. Therefore, before preparing the samples, the powders are analyzed by X-ray diffraction. With this analysis it is verified whether the iron powder contains only the ferritic phase and whether the <NUM> steel powder contains only the austenitic phase. If phase impurities are present, these are quantified and considered in the calculation of the desired amount of Austenite for the reference samples.

It is therefore necessary to proceed with the mixing of the powders <NUM>. To proceed with the mixing of the powders it is necessary to use known quantities thereof.

As regards the weighing of powders, in a preferred aspect of the invention, an analytical balance (VWR®) with a resolution of <NUM> was used.

A third component that is mixed with the powders is a binder. The binder must be amorphous and possess excellent binding properties as well as be stable to X-rays. In a preferred aspect of the invention the wax-based binder CEREOX® <NUM> (Fluxana®) commonly used to form pellets is preferred. Generally <NUM> part of binder is used for <NUM> parts of metal powders.

In a preferred aspect of the invention, the iron powder is weighed in a nacelle and then the contents are transferred into a zirconia jar. The residual powder in the nacelle is then weighed again in order to calculate the exact amount of powder of iron used. Generally the quantity of iron powder to be weighed is between <NUM> and <NUM>. With the same procedure, the austenitic steel powder is weighed in a new nacelle and then the contents are transferred into the same jar containing the iron powder. Generally the quantity of iron powder to be weighed is between <NUM> and <NUM>. The amorphous binder is then weighed as described above.

The mixing of the powders can take place in any way known in the art. In a preferred aspect of the invention, a laboratory mill was used, preferably a vibromill. In particular, in a preferred aspect of the invention a vibromill MM400 (Retsch®), equipped with zirconia jars and grinding spheres of the same material; the diameter of the spheres was <NUM>. As known to those skilled in the art, mixing times must be proportional to the amount of sample inserted in the jar. For <NUM>-<NUM> total grams (binder plus Iron and Austenite powders) it is advisable to mix for <NUM> minutes at <NUM>.

At the end of the mixing, the grinding spheres are removed and the contents of the j ar are transferred to an airtight container on which the lot number and the Austenite content are indicated, calculated on the basis of the exact quantities of ferritic and austenitic powder inserted in the grinding jar.

The next stage involves the preparation of the sample and, in particular, the verification of its homogeneity <NUM>.

The sample can be prepared according to any of the ways known in the art. In the present invention a mold compatible with a Vaneox® <NUM> ton manual hydraulic press was used. (Fluxana®). The amount of mixture that is used varies between between <NUM> to <NUM>. Time and pressure are characteristics well known to those skilled in the art. Generally a pressure between <NUM> ton / cm<NUM> and <NUM> ton / cm<NUM> is applied for a time between <NUM> and <NUM> seconds.

The first pellets were prepared using <NUM> of powder and applying a pressure of <NUM> tons for <NUM> seconds. The press manufacturer, however, recommends a maximum pressure of <NUM> tons for samples with a diameter between <NUM> and <NUM>, as in our case (Ø <NUM>).

It was therefore decided to limit the pressure to <NUM> tons and reduce the amount of dust. The thickness of the tablets obtained with the new parameters (<NUM>, <NUM> ton, <NUM> seconds) is approximately <NUM>. Pellets prepared with <NUM> of powder, <NUM> tons for <NUM> seconds have one thickness of about <NUM>. The maximum pressure must be reached gradually in order to reduce the air trapped in the sample, which could cause flaking of the surface layer.

Once the resulting pellet has been extracted from the mold, the tablet is applied in an amorphous support, for the protection / storage of the sample and greater ease of use by the operator.

This is analyzed by diffractometry. In a preferred aspect of the invention, an Explorer® (GNR®) powder diffractometer was used, equipped with both a tube with a chrome anode and a tube with a molybdenum anode. The diffractometer was checked using the NIST SRM <NUM> and SRM <NUM> reference standards by X-ray diffraction.

The homogeneity of the tablet / sample is checked by carrying out, on both sides, three measurements along three different directions chosen at random. For each batch, a tablet is divided in half to carry out a further homogeneity check inside. The sample is defined as homogeneous if the standard deviation of all measurements is less than <NUM> vol%.

From the analysis of the sample, the Austenite <NUM> content is finally verified.

Below is an example of a preferred embodiment that will make it even clearer how the invention allows to achieve the intended objects. This example should not be construed in a limiting sense and the invention thus conceived is susceptible of numerous modifications and variations all falling within the scope of the present invention as it results from the attached claims.

The iron and <NUM> steel powders were analyzed with a powder diffractometer using a chromium anode (Cr Kα <NUM>Å) and subsequently with a Molybdenum anode (Mo Kα <NUM>Å). The quantification of the phases starting from the diffractograms collected with the instrument was carried out according to the ASTM E975 standard.

<NUM> of iron powder were weighed into a nacelle and transferred to the zirconia jar. The empty nacelle was weighed and the weight of the powder left in the nacelle was noted (<NUM>). The weight of the residual powder was subtracted from the weighing and the net weight of the iron powder transferred into the jar was noted (<NUM>).

<NUM> of <NUM> steel powder was weighed in a new nacelle, which was transferred to the same jar in which the iron powder was placed. As previously done, the empty nacelle was weighed and the weight of the remaining powder was noted in the nacelle (<NUM>). Eventually the net weight of the steel powder transferred to the jar was <NUM>. <NUM> of Cereox were then weighed and placed in the grinding jar together with <NUM> zirconia spheres with a diameter of <NUM>.

After closing the jar, mixing was started in the vibromill for <NUM> minutes at <NUM>. At the end of mixing, the grinding balls were removed and the contents of the jar were transferred into an airtight container on which it was noted the batch number and the Austenite content calculated on the basis of the exact quantities of ferritic and austenitic powder inserted in the grinding jar.

<NUM> of powder-binder mixture was weighed and transferred into the mold. The piston was inserted into the mold, and it was put into the press by applying a pressure of <NUM> tons for <NUM> seconds. The thickness of the tablets obtained with these parameters is approximately <NUM><NUM>.

Once the sample has been extracted from the mold, the Austenite content is verified by analyzing each sample by X-ray diffraction. The sample is analyzed three times per side.

The particle size of the starting powders and the dimensional homogeneity of the two phases are the two fundamental parameters to have a linear and direct response, between the theoretical content of Austenite (weighted quantity) and the content observed by X-ray diffraction: using Austenite and Ferrite with average particle size between <NUM> and <NUM>, a linear correlation is obtained with an angular coefficient substantially equal to <NUM> as shown in <FIG>. A linear and direct correlation is observed (y = <NUM>. 9935x + <NUM>; R<NUM> = <NUM>) by indicating that the Austenite value measured for X-ray diffraction corresponds to the expected value from the weighings and therefore no calibration between the expected value and the measured value is necessary.

The graph in <FIG> instead shows the correlation between the reference samples prepared with austenic and ferritic powder with average particle size less than or equal to <NUM> in the laboratory (triangular points) and those of real samples (round points).

This graph shows how the response of the reference samples prepared for weighing give an in-line response (y = <NUM>. 9957x + <NUM>; R2 = <NUM>) with the response of real samples prepared by heat treatment and whose theoretical value was determined through an interlaboratory round-robin procedure. This correspondence, found for X-ray diffraction measurements carried out with a Molybdenum anode, was not observable with the reference samples currently available on the market.

By essentially repeating the example according to the invention, but using a particle size of Austenite and Ferrite greater than <NUM>, a linear response is no longer observed.

Claim 1:
Method for preparing reference samples for the determination of residual Austenite by X-Ray diffraction characterized in that it comprises the following steps:
a. Characterizing (<NUM>) the phase purity of iron powders having average particle size between <NUM> and <NUM> and greater than <NUM>% purity and austenitic steel powder with average particle size between <NUM> and <NUM> and purity greater than <NUM>% by x-ray diffraction;
b. Mixing (<NUM>) for a suitable mixing time said powders and an amorphous binder by preparing a sample, wherein the mixing time is proportional to the sum of the amounts of powders and amorphous binder which make up the sample;
c. Verifying (<NUM>) that the sample is homogeneous by performing <NUM> measurements of the austenitic phase by X-ray diffraction in such a way that, on both sides of the sample, three measurements are taken along three different directions chosen at random, with the sample being homogeneous if a standard deviation of the measurements is less than <NUM> vol% so that the sample has, between weighted Austenite content and content observed by X-ray diffraction, a linear and direct correlation with an angular coefficient substantially equal to <NUM>; and
d. Checking (<NUM>) the content of residual Austenite in said sample by X-ray diffraction.