NO136293B - - Google Patents

Description

Weldable, corrosion-resistant steel.

This invention relates to weldable, corrosion-resistant steel, preferably cast steel.

When producing e.g. cast blade blanks for impellers in water turbines, where a combination of good strength properties and resistance to corrosion as well as weldability is desirable, so-called 13 Cr steel has been used in most cases up to now.

The usual 13 Cr steels have their directional analyzes within the limits C 0.10-0.25 per cent, Cr 11-14 per cent, Ni 0-1.5 per cent, with usual Si and Mn content. The steel is normally used after normalization and subsequent annealing or after hardening with subsequent tempering at a relatively high temperature (tough hardening). In normal condition, the steel has a breaking strength in the range 60-70 kg/mm<2>, a tensile strength in the range 45-55 kg/mm<2> and an impact strength at room temperature according to Carpy V in the range 2-5 kg/cm <2>. The strength values within the mentioned areas are dependent on the heat treatment to which the steel is subjected.

In terms of structure, steel with a carbon or nickel content within the upper part of the above-mentioned analysis values can be described as martensitic, that is to say completely hardenable. Steel with carbon or nickel content within the lower part of the analysis range is ferritic-martensitic steel. This means that such steel cannot be completely austenised at any temperature, and therefore also cannot be completely imparted with a martensitic structure during hardening. The amount of ferrite in such a steel with a typical ferrite-martensitic structure of the above-mentioned type can be up to, e.g. 30-50 percent

As a result of the large amount of metallic alloy blanks, the hardenability of the steel is very good. This therefore also applies to the preconditions for partial hardening of the ferrite-martensitic type. The critical cooling rate is very low, and the steel can be described as "air-hardening", even though it is normally hardened in oil during quench hardening.

As a result of the strong tendency to harden, the weldability of the steel is poor. This is due to the fact that during welding a brittle martensitic transition zone is obtained, or - in the case of ferritic-martensitic steels - a partially martensitic zone on both sides of the weld seam. As a result of the steel's high hardenability, this zone cannot be significantly eliminated when welding at an increased working temperature (preheating). If the workpiece is allowed to cool to room temperature after welding, the martensite structure will appear in the transition zones, even if the workpiece has been preheated.

The formation of martensite, which in itself provides brittle material parts, becomes particularly dangerous with regard to crack formation, as there is a risk that hydrogen will make the material brittle in the transition zones while the welding is taking place. The hydrogen in the welding material diffuses to a large extent into the surrounding base material parts, so that the transition zones are made even more brittle, with the consequence that the formation of welding cracks, e.g. in the form of fusion boundary cracks along the weld seams are very difficult to avoid.

One has therefore resorted to extremely complicated operations in connection with the welding of such steels, e.g. an initial basic welding of all welding joint surfaces with austenitic welding electrodes, usually of the type 19 Cr/9 Ni/2.5 Mo. The basic welding is carried out at a working temperature of 200-300° C, after which a suitable annealing follows at a high temperature. A "subsequent heat treatment" means that the workpiece at the working temperature is directly fed into a furnace, without first having the opportunity to cool at a lower temperature (so-called "direct annealing"). As a rule, the weld seam is also finished with austenitic electrodes of the aforementioned type and at an increased working temperature, after which, if possible, a direct annealing also follows. In this case, however, it is less required to carry out annealing immediately after the welding, after which the basic welding allows the final welding to be carried out with austenitic electrodes against an austenitic substrate. For practical reasons, it is usually not possible to carry out the final annealing immediately after welding without intermediate cooling taking place.

The reason why austenitic electrodes are used for such welding-technically difficult steels is partly due to the usually good shape change ability of the austenitic welding material, and partly to the fact that hydrogen dissolves significantly more easily in such welding materials than in the ferritic materials. This reduces the risk of material embrittlement due to hydrogen in the transition zones.

The above-mentioned difficulties due to the 13 Cr steels' great tendency to harden also apply to steels of the ferritic-martensitic type. There is also a very strong increase in grain size in the ferrite phases of the transition zones. This coarse-grained ferrite is usually also distinctly brittle.

As austenite's ability to change shape is greater than martensite's, and at the same time that austenite is not so prone to embrittlement due to hydrogen, the possibility of composing the steel so that a certain amount of residual austenite is obtained in the transition zones has been fixed. This can conceivably be achieved by further addition of austenite-forming alloy blanks, e.g. C, Mn, Ni or N.

The investigations carried out have shown that of these alloy blanks, nickel of several years' use is preferable to the other blanks, which are essentially austenite-stabilizing blanks. An excessively large increase in the carbon content is inappropriate,

for reasons of impact resistance, among other things. This

<:>can also in some cases apply to larger increases in the manganese or nitrogen content.

However, it has been shown that the increase of

The Ni content leads to the desired result,

i.e. a significant amount of retained austenite i

the transition zones within the weld seams

with better shape-changing ability in these zones as a result, and reduced susceptibility to hydrogen embrittlement. The high nickel content also contributes very strongly to improvement

of the impact strength, compared to the values that apply to normal 13 Cr steel.

In the best case, the latter show a lower cover temperature in the Charpy V test at around 0° C, while the new steels have a significantly lower cover temperature.

It should be mentioned in this connection that the traditional austenitic stainless steels. which contains 18 percent chromium and 8 percent nickel and which can. welded without. preheating and subsequent annealing are not suitable for use where good strength properties are required, as they have a breaking limit of around 50-60 kg per mm-' and a tensile limit of approx. 18-25 kg per mm<2>. Constructions made of such material are therefore very heavy and also extremely expensive, partly as a result of the large weight and partly as a result of the material's high price per unit. weight unit.

A weldable, hardenable and corrosion-resistant steel with 11-14 percent chromium content and with an increased tendency to form residual austenite in the transition zones within the weld seam is distinguished according to the invention by the following composition of the content of the alloying elements.

C 0.03-0.25%

Say 0.10-0.70%

Mn : 0.25-2.00%

Nine 4-8%

where the low carbon contents must be used when the Cr content is low and where the ratio between the included amounts of nickel equivalent calculated as % Ni + 0.5 x % Mn and chromium equivalent calculated as (% Cr — 15 x % C) + 1.5 x % Si is between 0.4 and 1.0 and that the alloy after heating

to 1000° C within 3 hours, slow cooling in air to room temperature and heating to 575-600° C within 1 hour and slow cooling to room temperature contains between 15 and 40 percent austenite.

The fact that the coal content must be kept low with a low chromium content is connected with the requirement for corrosion resistance. The carbon binds the chromium (= 15 x the carbon content) in carbides, so that the chromium content in the base material decreases. With a high carbon content and low chromium content, there is a risk that the chromium content in the base material is below the limit value for what corresponds to good corrosion resistance.

A certain ratio between the nickel and chromium equivalents must exist within the limits of the analysis, if the steel is to obtain the desired residual austenite content after the heat treatment explained above. This ratio must be between 0.4 and 1.0.

It has been shown that the steel according to the invention contains in the transition zone at the weld seam an amount of residual austenite which is sufficient for the welding to be carried out without preheating and without immediately subsequent heat treatment. In addition, the steel's strength and impact resistance properties are better than in the case of the 13 Cr steel.

The following limit values are preferably chosen for the alloying elements: C 0.05-0.10%

Say 0.20-0.40%

Mn : 0.80—2.00%

Nine 5-7%

It has been shown that nitrogen has no major importance as an austenite-stabilizing alloy element in this type of steel, but that a nitrogen content of up to 0.25 per cent, e.g. within the range of 0.05-0.20 per cent has a beneficial effect on the impact strength of the steel, probably as a result of nitrogen contributing to a smaller grain size.

The weldability of the aforementioned steels can be further improved by adding smaller amounts of strongly carbide-forming materials, preferably niobium or tantalum, but also zirconium, vanadium or tungsten in amounts up to a total of approx. 0.25 per cent, preferably 0.05-0.20 per cent. As far as it appears from the investigations, niobium and/or tantalum addition also has a beneficial effect on the impact strength, which does not apply to the same extent for the other carbide-forming items mentioned .

The purpose of adding strong carbide

forming alloy blanks is to be found in that the poorly soluble, stable carbides bind during the rapid austenisation process that takes place during welding a significant part of the carbon content, so that this carbon does not dissolve in the austenite. Thereby, the austenite's carbon content is relatively low, which results in less martensite hardness and thus less risk of cracking in the transition zone.

The strength of the new steel depends on the heat treatment to which it is subjected. The breaking point is usually within the range 85-95 kg/mm<2>, the tensile limit within 55-75 kg/mm-, in addition to the above-mentioned good impact strength which in the range of the high values lies within the interval 11-13 kgm/cm< 2> with a lower cover temperature lower than ^-80° C and in many cases within the range —100/ — 120° C.

The normal heat treatment for the new steels consists of an oil hardening

around 1000° C with subsequent tempering at 600° C. Due to the composition of the steel, however, the different possibilities for the heat treatment can be varied within wide limits, so that e.g. by increasing or lowering the tempering temperature, a greater or lesser amount of residual austenite is obtained.

The steels, which in and of themselves should be described as martensitic, can in certain heat treatment conditions be called "martensitic-austenitic".

Example 1.

A steel with the following content of alloying elements: and the rest Fe with common impurities was used for casting a workpiece with the following dimensions:

The subject received the following heat treatment:

'Heating to 1000° C.

Constant temperature during 3 hours. Slow cooling in air to room temperature.

Heating to 600° C.

Constant temperature during 1 hour. Slow cooling in air to room temperature.

Two such blanks were clamped together and cold welded with an electrode which produced the following weld material:

and the rest Fe with common impurities.

The subject had the following strength data in unaffected base material: 8 0.2 = 75 kg/mm2

8 B =93 kg/mm<2>

8 5= 17.0%

rp = 55.2%

No cracks were found in the transition zones at the weld or in the weld itself. The zone material contained approx. 25 percent residual austenite.

Example 2.

A steel with the following composition

and the rest Fe with common impurities was used for casting a blank with the same dimensions as the blank according to example 1 and received the same heat treatment and was welded in the same way. The subject's firmness values were as follows: 8 0.2 = 50.3 kg/2

8 B = 87.9 kg/mm-'

8 5= 19.0%

<q>> = 51.0%

The blank had the same good weldability and the transition zones contained approximately 30 percent residual austenite.

Example 3.

A steel with the following composition

and the rest Fe with common impurities was used for casting a blank which had the same dimensions as according to example 1 and which then received the same heat treatment and was welded together in the same way. The strength values were as follows: 8 0.2 = 73 kg/mm<2>

8 B =100 kg/mm<2>

8 5 = 15.0%

cp = 52.3%

The workpiece had the same good weldability and the transition zones contained approx. 30 percent residual austenite.

Example 4.

A steel with the following composition:

and the rest Fe with common impurities was used to cast a blank as in Example 1, which was then heat treated and welded in the same manner as above.

The following strength values were obtained: 8 0.2 = 72 kg/mm<2>

8 B = 99.4 kg/mm<2>

8 5= 16.0%

cp = 53.8%

The workpiece had the same good weldability, and the transition zones contained approx. 30 percent residual austenite.

Example 5.

A steel with the following composition:

and the rest Fe with common impurities was forged into a round blank with a diameter of 20 mm and given the same heat treatment as above.

The following strength values were obtained: 8 0.2 = 65 kg/mm<2>

8 B = 103 kg/mm<2>

8 5 = 18.0%

cp = 60.4%

Example 6.

A steel with the following composition:

and the rest Fe with common impurities was used for casting a 90 kg test bar blank.

The subject was exposed to the following heat treatment: Heating to 1000° C. Constant temperature during 3 hours. Slow cooling in air to room temperature. Heating to 590° C. Constant temperature during 3 hours. Slow cooling in air to room temperature.

The subject's firmness values were as follows:

8 0.02 = 62.5 kg/mm<2 >8 B = 96.2 kg/mm<2 >8 5 = 17%

cp = 57%

A 20 mm deep V-groove was milled into the workpiece and filled with welding material with the following composition.

and the rest Fe with common impurities.

The welding was carried out without preheating. 2 tensile tests were taken out above the weld. During testing, the fracture occurred in the middle of the welding material. The material was severely deformed right up to the melting zone boundary. No signs of cracks or breaks could be seen in the melting zone boundaries or in the heat-affected base material. The residual austenite content in the base material varied between 25 and 35 per cent, and the same amount was also found in the transition zones at the weld seam.

Example 7.

A steel with the following composition:

and the rest Fe with the usual impurities was used for casting a 90 kg sample blank.

The workpiece was heat treated in the following way: Heating to 1000° C. Constant temperature during 3 hours. Slow heating in air to room temperature return.

Heating to 575° C. Constant temperature during 3 hours.

Slow cooling in air to room temperature.

The following firmness values were measured:

8 0.2 = 69.5 kg/mm<2 >8 B = 93.0 kg/mm2 8 5 =19%

q, = 60%

The retained austenite content was 18 percent.

Example 8.

A steel with the following composition:

and the rest Fe with common impurities was used for casting a 90 kg test bar blank. The heat treatment was as per example 7.

The following strength values were obtained: 8 0.2 = 69.5 kg/mm<2>

8 B = 96.2 kg/cm2

8 5 = 19%

cp = 59%

The retained austenite content was 22 percent.

Claims (4)

1. Weldable, hardenable and corrosion-resistant steel with 11-14% Cr content and with an increased tendency to form residual austenite at the transition zones of a weld, characterized by the following analytical limits for the alloying elements: C : 0.03 — 0.25% Si : 0.10 — 0.70% Mn : 0.25 — 2.00% Ni : 4 — 8% where the low carbon content must be used when the Cr content is low and where the ratio between input amounts of nickel equivalent calculated as % Ni + 0.5 x % Mn and chromium equivalent calculated as (% Cr — 15 x % C) + 1 .5 x % Si is between 0.4 and 1.0, and that the alloy after heating to 1000° C during 3 hours, slow cooling in air to room temperature and heating to 575 — 600° C during 1 hour and slow cooling in air to room temperature contains between 15 and 40 percent austenite. 2. Steel according to claim 1, characterized by the following analysis limits for the alloying elements: C : 0.05 — 0.10% Si : 0.20 — 0.40% Mn : 0.80 — 2.00% Ni : 5 — 7% 3. Steel according to claim 1, characterized in that it also contains up to 0.25% N. 4. Steel according to claim 1 or 2, characterized in that it also contains 0.05-0.25 percent niobium or niobium and tantalum.