DE4009239C1 - - Google Patents

Description

The invention relates to a method for producing work pieces made of 9 to 12% martensitic temperable chrome steel according to the preamble of claim 1.

One such chromium steel is the 12% Cr steel 1.4914, which because of its low swelling rate under neutron radiation Manufacture of cladding tubes and fuel assemblies for nuclear reactors is used and as a structural material in future Fusion facilities is provided.

The processes used in the manufacture of such components can be generally described by a sequence of Cold forming steps and intermediate heat treatments, which in the Range of the usual tempering temperature can be carried out. Finally there is a final heat treatment, the hardening and tempering, the so-called remuneration includes and another Cold forming, in which the workpiece to the exact final dimension brought.

The cold working is generally carried out with an average degree of cold working ε in the range from 15 to 50% single step. The annealing between the individual cold forming steps is carried out at temperatures down to below the α / γ conversion (A _c1 start) between approximately 600 and 780 ° C.

The heat treatment consists of a hardening annealing (austenitization) at a temperature above the transition temperature A _c1 up to γ / δ transformation temperature approximately in the range 950 to 1150 ° C. and a further tempering annealing at the specified temperatures.

In the manufacture of cladding tubes and fuel boxes steel 1.4914 according to the described procedure was common Damage due to cracking, making a considerable Part of the workpieces either according to the final test procedure or already discarded as a committee during production had to become. In addition, some of the workpieces reached not the specified mechanical strength. They watched Damages are obviously material-specific, because the austenitic steel 1.4981 and Zircaloy 4 were made after the process described without problems.

The damage in the manufacture of cladding tubes and fuel assemblies can be achieved by varying individual ver driving parameters, such as B. the temperature during the heat treatments or by changing the cooling conditions avoid.

The invention has for its object to provide a method which is the manufacture of crack-free and mechanical heavy-duty workpieces made from 9-12% martensitic temperable chrome steels through a series of cold forming steps and temper annealing without being allowed during machining Semi-finished products or after completion of processing Completed workpieces sorted out as rejects Need to become.

Those proposed according to the invention to achieve the object Measures are in the characterizing part of claims 1 and Called 2.

The average degree of cold deformation ε in% is shown by the cross change in section ΔF / F₀ · 100 defined. This definition of ε, which is common for round hammers and pulling, is also used for Cold rolling and pilgrims on the basis. With the latter In general, only the decrease in thickness is used  d to determine the nominal cold deformation according to (Δd / d₀ · 100) specified, the changes in width however are not taken into account. The transition from this nominal for effective deformation in which the spreading into the If the calculation is included, the ver degree of shaping, d. that is, the effective deformation is smaller.

The invention is based on the observation that material consolidation through the cold forming to one across the cross section leads to inhomogeneous deformation. In the core zone and in the outermost edge zone decreases with the subsequent one Tempering heat treatment the hardness of the semi-finished product through recrystallization while the zone in between remains firm and has a remuneration structure.

In the further course of the cold deformation, the recrystallized one expands Core zone at the expense of the between core and peripheral zone lying areas without these areas completely disappearing. The hardness of the recrystallized core and peripheral zone rises slowly during processing; in between areas, however, the hardness falls with every cold deformation step, but maintaining the remuneration structure remains.

After several cold forming and heat treatment steps the hardness of the crystallized zones and the areas approach with remuneration structure, however, remains one two-phase state exist.

The inhomogeneous deformation described is the real one Cause of the cracking and the lack of mechanical strength in the production of cladding tubes and boxes. During the annealing treatment show the different zones and areas a different recovery behavior. Annealing in The usual temperature range does not fully build the solidification from, so that depending on the local deformation rate  and the annealing temperature is the critical degree of deformation for the recrystallization either in the first or in the subsequent deformation steps is achieved. Out of it the observed locally progressive recrystallization is explained front.

The first cold deformation step remains significantly below the critical cold deformation rate of 40% in the case of the subsequent annealing treatment Recrystallization can be observed, so is the structure of the material recovered after the annealing treatment; it results a remuneration structure.

However, it has been found that the total cold deformation is approximate specified as the sum of the individual cold forming steps can be.

This means that with subsequent cold working steps the critical degree of cold deformation is partially exceeded and that then the effects described occur, too much different strengths and thus to cracks in the further Lead procedure. The cause of these cracks is not the low strength of the recrystallized material, but the different strengths of recrystallized and not recrystallized structure.

As mentioned, a cold working step leads to one the cross section of the semi-finished product to be processed inhomogeneous deformation. Has this inhomogeneous deformation an inhomogeneous solidification.

The mean degree of cold deformation ε is therefore not suitable for completely describing a cold deformation step. To describe the cold deformation, the maximum degree of cold deformation ε _max and the minimum degree of cold deformation ε _min must therefore be introduced. The degrees of cold deformation ε _min and ε _max indicate the range of cold deformation that is achieved with a deformation step with an average degree of cold deformation ε according to the definition mentioned at the beginning.

In the method according to the invention, the term average degree of cold deformation ε is abandoned in favor of a cold deformation range within the basic values of ε _min and ε _max .

The local one in the material to be processed Cold deformation is proportional to a change in hardness ΔHV. It applies: ΔHV = m · ε, where m is the mean hardening coefficient represents.

By determining the change in hardness of samples that are to be processed Semi-finished product or pre-profile after individual cold forming steps taken at various points in the cross-section the local degrees of cold deformation be determined.

The technical problems mentioned, such as cracking and insufficient mechanical strength of the finished workpiece, occur when, in one of the process steps cold forming / annealing, the maximum degree of cold deformation ε _max exceeds the critical degree of cold deformation and the minimum degree of cold deformation ε _min falls below the critical degree of cold deformation and if thereafter Another cold deformation is carried out without further measures.

In this case, partial recrystallization occurs very different strengths in the individual cross cut areas that are not compensated by the annealing and the subsequent cold forming Cause damage.

The described case that the critical degree of cold deformation lies in the range between ε _min and ε _max during processing occurs with high probability when processing semi-finished products, provided that the degree of cold deformation and the tempering temperature cannot or cannot be coordinated. In this case, it is proposed according to the invention to carry out a complete tempering treatment, consisting of hardening and a subsequent tempering annealing, instead of the annealing.

With this remuneration treatment, the material becomes homo again genized and shows uniform in all areas of the cross-section Strength values.

However, the degree of cold deformation and the tempering annealing temperature are preferably coordinated with one another such that the critical degree of cold deformation never lies in the range between ε _min and ε _max , or, in other words, that the maximum degree of cold deformation ε _{max is} either below or the minimum degree of cold deformation ε _{min is} above the critical degree of cold deformation.

As described, all process parameters can be sampled on appropriately pre-processed semi-finished products determined, coordinated and for the machining process be determined.

The invention is described below with reference to figures and examples explained in more detail.

Fig. 1 shows the influence of cold working and subsequent heat treatment on the hardness (cold working by hammering),
Curve A: reference condition, after tempering treatment 1075 ° C - 30 min V / V and 700 ° C - 2 h V / V,
Curve B: Condition after annealing (reference condition + cold working + annealing with a duration of 2 hours) depending on the KV degree of annealing temperature a) 700 ° C, b) 750 ° C, c) 780 ° C;

Fig. 2 shows the hardness curve as a function of cold working (KV) and tempering treatment to determine the mean hardening coefficient m;

Fig. 3 shows the solidification, depending on the degree of cold deformation in a plurality of austenitic steels;

Fig. 4 shows the hardness curve of round hammered samples over the cross section depending on the KV degree;

Fig. 5 shows the hardness curve around hammered samples from austenite 1.4981 and Zircaloy 4, depending on the degree of deformation;

Fig. 6 shows the effect of sequential cold deformation / heat treatment steps on the curing behavior in the steel 1.4914;

Fig. 7 shows the HV1 hardness curve over sample thickness and width in dependence on the nominal degree of cold deformation during cold rolling;

Fig. 8 shows the HV30 hardness curve as a function of cold deformation by cold rolling and the tempering temperature using the example of a flat bar 20 × 6 mm with the reference state 1075 ° C - 30 '+ 700 ° C - 2 h. There is a remuneration structure in area V and a recrystallized structure in area RE.

Unless another material is mentioned, everyone is Figures of the material RNO-D (1.4914) Chg. 53093.

Example 1

Rod-shaped material with an initial diameter of ⌀12 mm from material 1.4914, batch 53093 was used in the tempered initial state (1075 ° C 30 ′ V / V + 700 ° C 2 h V / V). The material was cold formed in 10% increments by round hammering up to 80%. After each deformation step, 4 disks were cut from the rod, on which the hardness HV 30 was measured. There was a solidification curve, as shown by the closed symbols in FIGS. 1a to 1c. Subsequently, one disk was annealed from each deformation step at 700, 750 or 780 ° C. for 2 h and the hardness measured again and the data in FIGS . 1a to 1c entered.

The resulting results can be as follows sum up:

• - The hardness increase HV caused by cold deformation is in a first approximation directly proportional to the degree of cold deformation ε [%], which is defined by the change in cross-section ΔF / F₀ · 100, i.e.: ΔHV = m · εFor Fig. 2, the hardening coefficient can be m Take the value of ≈1.55.
  The flattening of the hardening curve at 80% KV seems to indicate an beginning tempering effect, because during round hammering from 70 to 80% KV the rod warmed up so much that it was no longer possible to work without protective gloves.
  It is interesting and noteworthy that in earlier investigations of the hardening behavior on austenitic steels 1.4970, 1.4981 and 1.4988, significantly higher hardening coefficients m were measured ( FIG. 3) and these decrease with increasing degree of KV. If one calculates with an averaged value m, then it is almost three times that found in martensite 1.4914.
• - According to FIGS . 1a to 1c, softening is caused by a subsequent tempering heat treatment between 700 and 780 ° C. As expected, it rises with a higher tempering temperature. The remaining hardness can also be represented according to FIG. 2 for the edge zone as a linear function of the original cold deformation.
• - In the core zone of the samples occurs after annealing at 700 ° C / 2 h from a KV degree of 60% and after annealing at 750 or 780 ° C / 2 h above 40% KV a strong softening on (due to recrystallization, like the metallographic Findings showed).

Example 2

First, the exact hardness curve over the sample cross-section was measured with HV 0.1 and HV 1 on three round-hammered samples with 15, 30 and 70% cold deformation ( Fig. 4a-c). According to FIG. 4b, a KV degree of 30% already causes a clearly inhomogeneous deformation across the sample cross section, with a very strong increase in hardness in the core zone area and at the extreme edge of the sample. This tendency of increased deformation inside the sample increases with increasing average KV degree ( FIG. 4c). With 1.4914, round hammering leads to uneven deformation across the cross-section. This is apparently a material-specific effect, because with comparable deformation of austenitic steel 1.4981 and Zircaloy 4 by round hammers, as shown in FIG. 5, even with degrees of KV of 50% and above there were no pronounced deformation inhomogeneities.

The impact of such inhomogeneous deformation on recovery behavior during subsequent annealing treatment can be done in metallographic Overview pictures are tracked. It can be clearly an inner core zone and an outer peripheral zone Coarse structure and an intermediate ring zone with fine structure detect.

The hardness values found in the ring zone normally correspond to that temper hardness observed at this temperature and the structure is identical to a normal remuneration system gap.

In the core zone is within the original austenite grains the fine structure, consisting of martensitic slats and slats, no longer present, the structure appears recrystallized. In order to clarify the facts, using Target preparation using electron microscopic samples a total of 66% cold worked sample after subsequent Tempering heat treatment taken at 730 ° C / 2 h. The electrons microscopic overview of the ring zone shows the typical sub-grain formation for a remuneration structure, the original slatted structure is largely dissolved and a strong decoration of the sub-grain boundaries with M₂₃C₆ excretions occurs. In addition, the subgrains are the largest Part dislocation-free. This allows the observed reduction in Firmness can be explained. In the transition area to the core zone you can see that in one section the subgrains are totally dissolved and a complete recrystallization occurred  is. Only those originally formed at the sub-grain boundaries Eliminations still exist.

Example 3

A round rod with 12 mm ⌀ and the initial heat treatment 1075 ° C 30 ′ V / V + 700 ° C 2 h V / V was increased eight times by 30% each Cold-formed by round hammering and after each deformation step annealed at 730 ° C / 2 h (stage I-VIII). After every One stage was cut off, examined metallographically and measured the hardness.

After stage I, the metallographic examination reveals a continuous tempering structure, the hardness of which, as expected, HV30 = 252-261 corresponds to that in FIG. 1a ( FIG. 6, curve B). After the second cycle (stage II) - caused by the inhomogeneous deformation - recrystallization occurs in the core zone and the outermost edge zone, which leads to a corresponding reduction in the hardness (HV30 = 180-200) and strength of the material; the ring zone with the typical compensation structure remains fixed ( Fig. 6, curve C).

Example 4

The previous examples relate to cold hammering. It Therefore, the question arises whether technically very much More common types of deformation, such as cold rolling, cold drawing and cold pilgrims, also such strong variations in deformation or not.

For a direct comparison of the deformation caused by cold hammers and cold rolling became a flat bar of the same material with the Dimensions 6 × 20 mm on a roll stand by 30, 50, 70 or 80% deformed.  

The HV30 impressions are not suitable for capturing subtle variations in the deformation behavior. For this reason, low load hardness values HV1 were measured over the sample thickness and width of the rolled samples. The results are shown in Fig. 7a + b, they also indicate inhomogeneous deformation. For KV grades 50% there is a stronger deformation in the middle of the sample, which is in line with corresponding measurements on 50% cold pilgered fuel box profiles. Hardness points positioned along the middle fiber also show a variation in hardness values across the sample width, which only disappears at relatively high degrees of deformation (<50%).

The influence of tempering heat treatment between 700 and 780 ° C on hardness and softening as well as on microstructural changes was investigated on the cold rolled samples. Fig. 8 shows the drop in hardness depending on the previous degree of deformation as a function of annealing temperature. By the way, metallographic examinations of these samples, in good agreement with corresponding statements for cold-hammered and subsequently tempered samples, show that recrystallization is observed on all the samples which have dropped to hardness values HV30210 after a two-hour annealing. On the other hand, this means that for a cold deformation of 30% in the covered temperature range, there is no recrystallization, but a tempering structure. The resulting statements about which critical degrees of deformation lead to recrystallization at the different temperatures are practically identical to the numbers given above for cold hammered samples.

Claims (3)

1. Method for producing workpieces from 9 to 12% martensitic, temperable chrome steel by machining a semi-finished product, in which the semi-finished product follows a sequence of steps

• a) cold working with an average degree of cold working ε,
• b) Annealing and subsequent to it
• c) is subjected to a heat treatment process instead of the last annealing,

characterized in that

• d) steps a) and b) are carried out with such a degree of cold deformation ε and at such an annealing temperature that
• d1) either the maximum degree of cold deformation ε _max occurring in steps a) below the critical degree of cold deformation for recrystallization at tempering temperature
• d2) or the minimum degree of cold deformation ε _min occurring in steps a) is above the critical degree of cold deformation for recrystallization at the annealing temperature.

2. The method according to the preamble of claim 1, characterized in that in the event that the maximum degree of cold deformation ε _max occurring in steps a) above and the minimum degree of cold deformation ε _min occurring in steps a) below the critical degree of cold deformation for recrystallization Tempering temperature is, instead of the tempering annealing heat treatment is carried out.