JPS583942A - Ni alloy with superior embrittlement resistance at intermediate temperature - Google Patents

Abstract

PURPOSE:To enhance the embrittlement resistance of an Ni-Mo alloy at intermediate temp. without deteriorating the resistance to hydrochloric acid by adding a very small smount of B to the alloy and restricting the contents of the inevitable impurities. CONSTITUTION:An Ni alloy with superior embrittlement resistance at intermediate temp. as well as high weldability and corrosion resistance is obtd. by providing a composition consisting of, by weight, 26-30% Mo, 0.01-2.0% Fe, 0.001-0.01% B and the balance Ni with inevitable impurities and by restricting the contents of Cr, Mn, Co, Si, C, P and S as the impurities to <=1.0% Cr, <=1.0% Mn, <=1.0% Co, <=0.1% Si, <=0.02% C, <=0.04% P and <=0.03% S. The Ni alloy does not cause annealing cracking when it passes through the temp. range of about 700-800 deg.C during annealing after cold working or welding.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an N1-based alloy that is resistant to so-called annealing cracks and has excellent intermediate temperature embrittlement resistance during annealing after cold working or welding.

In general, Ni-Mo alloys are corrosion-resistant alloys that are ideal for reducing corrosive atmospheres, and are known as alloys that have excellent corrosion resistance against non-oxidizing acids, especially hydrochloric acid. towers, tanks, and heat exchangers.

Or it is one of the alloys widely used as piping materials. These equipment parts are made by a combination of cold working such as bending and pipe drawing, and welding, but the residual strain and residual stress generated by such processing can be removed by annealing after the processing or during intermediate processing. It has typically been removed by application and then either further processed or used as a final product. however,
This annealing is normally carried out at a temperature of 1000 to 1150°C for several tens of minutes, but when the alloy part passes through a temperature range of 700 to 800°C while being heated to this temperature, It was known that the material undergoes significant embrittlement (intermediate temperature embrittlement) and therefore cracks under the action of high tensile residual stresses.

This annealing cracking is caused by a decrease in the bonding strength of the grain boundaries of this alloy at 1700-800°C, and also by a decrease in the bonding strength of the grain boundaries in this temperature range [N
Due to the precipitation of intermetallic compound Nj, 4Mo with o5e,
Traditionally, this type of stress is caused by the interaction between the effects of increased intracrystalline strength, increased hardness, and significantly reduced elongation, and high tensile residual stress during cold working or welding. The following methods have been considered as means for preventing the occurrence of annealing embrittlement cracking in alloys.

(a) By reducing the Mo content of the Ni-Mo alloy, the precipitation of the intermetallic compound Ni4Mo is suppressed, and the
A method of alleviating stress concentration at grain boundaries by keeping hardness low in the temperature range of 00°C.

(b) Since a certain amount of time is required for the precipitation of the intermetallic compounds Nj, 4Mo, the precipitation of Ni4Mo can be achieved by increasing the heating rate very quickly and rapidly passing through the brittle temperature region. A method to remove residual stress before it occurs.

(C) A method in which no residual stress remains during cold working or welding, or only compressive residual stress remains.

However, with the method shown in (a) above for reducing the Mo content, it is impossible to prevent the occurrence of annealing embrittlement cracking without impairing the excellent hydrochloric acid resistance of the Ni, -Mo alloy. In addition, the method shown in (b) above becomes extremely difficult to implement industrially when the target product becomes large. Furthermore, the method shown in (C) above eliminates all residual stress in all parts of the actual processed product. This is because it is difficult to predict and there are limited means to measure it, so it is extremely difficult to develop a processing method that does not leave tensile residual stress during cold processing or welding. However, there was a problem in that it could not be applied to all products.

From the above-mentioned viewpoint, the present inventors have found that when cold-worked parts or welded parts with high tensile residual stress are annealed at a temperature of 1000°C or higher by heating at a slow heating rate. In order to obtain an alloy with excellent intermediate temperature brittleness resistance and corrosion resistance that does not cause annealing cracking even in
N1-M was developed as a result of repeated research to improve the resistance to intermediate temperature embrittlement without losing its properties, based on alloys based on the N1-M series.

We have found that by including a small amount of B in the alloy and limiting the content of unavoidable impurities, the bonding strength of grain boundaries at 200 to 800°C is strengthened and the occurrence of cracks is suppressed. It is.

Therefore, this invention has been made based on the above knowledge, and uses a Ni-based alloy having excellent corrosion resistance against non-oxidizing acids, Mo: 26 to 30% (hereinafter, ``chi'' is % by weight). , Fe:
o, ol ~ 2.0%, B: o, oo1 ~ 0.01%, N] and inevitable impurities: remainder, 5 - and Cr as an inevitable impurity.

The contents of Mn, Co, Si, C, P, and S are as follows: cr: 1.0% or less, 14n: 1.0% or less, CO: 1.0% or less, si: 0.1% or less, C: 0.02% or less, p: o: 0.04% or less, S: 0.03% or less, and is characterized by providing excellent intermediate temperature brittleness resistance.

Next, in the N1-based alloy of this invention, Mo + F
The composition range of the e+B component is limited as described above, and Cr
.

The reason why the amounts of unavoidable impurities of Mn, Co, Si, C, P, and S are limited as described above will be explained.

(a) M.

The Mo component dissolves in N1, which is the base metal, and has the effect of improving the corrosion resistance of the alloy, especially hydrochloric acid resistance. Although it is an essential element, if its content is less than 26%, the desired effect cannot be obtained in the above action, while if it is contained in more than 30%, the hot workability and mechanical properties of the alloy at room temperature are affected. Because it starts to lower
Its content was limited to 26 to 30 inches.

(1)) Fe The Fe component has the effect of improving the hot workability of the alloy and the mechanical properties at room temperature, but if its content or amount is 0.0
If it is less than 1%, the desired effect cannot be obtained in the above-mentioned action, and in particular, only an alloy with extremely poor hot workability can be obtained.
On the other hand 2. ○Containing more than
It was limited to 2.0%.

(C) B The B component segregates at the grain boundaries of the alloy, strengthens the grain boundaries of the alloy, and prevents the occurrence and propagation of intergranular cracks.
It has the effect of increasing annealing cracking resistance, but if its content is less than 0.001%, the desired effect cannot be obtained, while if it is contained in excess of 0.01%, it may cause formation of grain boundaries. Since corrosion resistance will deteriorate and weldability will not be adversely affected, the content should be reduced to O, OO.
It was limited to 1 to 0.01%.

(d) C The C component is a harmful element that forms carbides of MO at the grain boundaries in the welding heat affected zone, deteriorating the corrosion resistance of the grain boundaries in this area and causing intergranular corrosion. amount is 0.0
Since this tendency becomes noticeable when it exceeds 2%, the content was limited to 0.02% or less. However, in order to ensure sufficient corrosion resistance, it is desirable that the C content be 0.01% or less.

(e) Cr, Mn, Co, Si, P, and S In addition to C, Cr, Mn, Co, Sj, and P are usually unavoidable impurities in Ni-Mo alloys.
These impurities cause reductions in the corrosion resistance, ductility, and weldability and workability of the alloy; 1vln, and 1 for co
% or less, 0.1% or less for sl, 0 for P
．． 0.04% or less S does not pose a practical problem if it is 0.03% or less, so the permissible range of its content is limited as described above.

Next, the Ni-based alloy of the present invention will be explained using examples and comparing with comparative examples.

Example 1 First, an alloy having the chemical composition shown in Table 1 was melted, hot-forged and hot-rolled using the methods normally used for Ni alloys, etc., and an annealed flat plate sample was prepared. was created. Sample numbers 1 to 3 are alloys of this invention containing a predetermined amount of component B, sample number 4 is a comparative alloy in which component B is not actively added, and sample number 5 is a comparative alloy containing 0. 01
This shows a comparative alloy containing B component exceeding %.

First, a high temperature tensile test was conducted on these five types of samples. In the high-temperature tensile test, the specimen was heated to a predetermined temperature within 5 minutes, held at this temperature for 20 minutes, and then subjected to a tensile test at a tensile rate of 1117 H/min. ) is constant. The test results are also shown in Table 1. From the high temperature tensile test results shown in Table 1, the present invention alloys 1 to 3, in which the B component was actively added, and the comparative alloy 5
is '700 compared to comparative alloy 4 which does not contain B component.
It is clear that the elongation at 50° C. and 50° C. is significantly improved, and the grain boundaries are strengthened when the B component is contained.

Next, the weldability due to the inclusion of B component and 20% HC1
The effect on corrosion resistance in boiling solution was investigated. The weldability test was carried out using a transparestrene test. To explain this test method, first, one side of a plate-shaped test piece is fixed on a bending block, and peak-on-plate welding is performed on the surface of the test piece using DC positive polarity 'I' IG. When moving to the center of the test piece, the test piece is momentarily bent along the bending block. At this time, the welding direction and bending strain direction are perpendicular to each other, and the bead on the test piece surface is A predetermined bending strain is applied to the part. At this time, the weldability is evaluated based on the length of the crack that occurs at the center of the bead.

A diagram showing the relationship between the amount of strain and the maximum crack length obtained by such a transpare strain test is shown in FIG.

From the results shown in FIG. 1, the alloy 1 of the present invention containing component B
There is no difference in the maximum crack length (the longer this is, the worse the weldability is) between alloys 3 and 4, in which the B component was not actively added, but the content of the B component exceeds 0.01%. In Comparative Alloy 5, the maximum crack length was longer, and this also confirmed that addition of excessive B component deteriorated weldability.

Furthermore, we also conducted corrosion resistance tests against normal corrosion.
This was done by keeping the specimens in a boiling solution of 20% HCl for 3 weeks. The results were as shown in Table 1. The results shown in Table 1 are the average values of the corrosion test results conducted on three samples. There is no difference in the corrosion depth between alloys 12-3 and comparative alloy 4, to which the B component is not actively added, and neither alloy causes intergranular corrosion, but the B component In Comparative Alloy 5, in which the content exceeds 0.01%, it was confirmed that the corrosion depth was slightly larger and intergranular corrosion occurred.

Example 2 Invention alloys 1 to 3 used in Example 1 and comparative alloy 4,
A pipe was made using the flat plate No. 5 using an upward non-filler automatic TIG welding machine. Outer diameter thus obtained:
A pipe with dimensions of 2'i', 2ifiφ x wall thickness + 2.2mm is drawn dry to make a pipe with dimensions of 2'7.0mmφ x 22mm, and this pipe is heated in an electric furnace in the atmosphere for 560 minutes.
℃ to 1066℃ in 2 hours and 30 minutes, 1066℃
Annealing was performed by taking the sample out of the furnace at ℃ and cooling it in air. Note that at this time, the temperature increase rate from 700°C to 750°C was 33°C/mm. A color check was performed on the annealed pipe to check for cracks. As a result, no cracking was observed in the 13-welded pipes made of Inventive Alloys 1 to 3 and Comparative Alloy 5, in which the B component was actively added. Cracks were observed in welded pipes made from Comparative Alloy 4 with no additives.

The results obtained in Examples 1 and 2 also show that the Ni-Mo alloy containing the B component within the scope of the present invention has good weldability and corrosion resistance, and has excellent intermediate temperature embrittlement resistance. It was confirmed that they have both.

As mentioned above, the 1h-based alloy of the present invention has a conventional Nj
-It has excellent corrosion resistance and weldability similar to Mo-based alloys, and has better resistance to intermediate temperature embrittlement than conventional Nj, -M alloys.
Since it has high resistance not found in o-based alloys, it can be annealed after cold working or welding without any consideration of residual stress, that is, even if it is in a high tensile residual stress state. , without reducing this stress or converting it into compressive residual stress, it is often necessary to particularly increase the temperature increase rate, and there is no cracking etc. by simply annealing it in a normal annealing furnace. It has industrially useful properties, such as being able to produce healthy, highly corrosion-resistant parts.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the amount of strain and the maximum crack length for alloys with various chemical compositions. Applicant Mitsubishi Metals Co., Ltd. Agent Tomi 1) Kazuo

Claims (1)

[Claims] Mo: 26 to 30%, Fe: 0.01 to 2.0%, B: 0.001 to 0.01%, N1 and unavoidable impurities: the remainder; Cr, Mn. The contents of Co, Si, C, P, and S are as follows: Cr: 1.0% or less, Mll: 1.0% or less, Co: 1.0% or less, 3i: 0.1% or less, C: 0.02% or less, P: 0.04% or less, S: 0.03% or less (weight%).