JPH0397823A - Ni-base alloy for cryogenic use - Google Patents

Abstract

PURPOSE:To provide the Ni-base alloy with high strength from an ordinary temp. to a very low temp. and to improve weldability by specifying the componental compsn. in an Ni-base alloy and specifying the relationship between the content of Nb+Ti and that of Mo. CONSTITUTION:The compsn. of an Ni-base alloy for cryogenic use is formed from the compsn. constituted of, by weight, <=0.03% C, <=0.25% Si, <=0.25% Mn, <=0.015%, P, <=0.015% S, 38.0 to 49.0% Ni, 12.0 to 20.0% Cr, 2.0 to 4.0% Nb, 0.7 to 2.5% Ti, 0.2 to 0.8% Al, >1.0 to 5.0% Mo and the balance Fe with impurities. Furthermore, 3.5%<= Nb+Ti <=[(Mo/4)+4.6]% is satisfied. In this way, the Ni-base alloy for cryogenic use having high strength of about >=80kgf/ mm<2> 0.2% proof stress at a room temp. and having excellent weldability to such a degree that HAZ cracks are not generated can be offered.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a Ni-based alloy suitable as a structural material for equipment that utilizes cryogenic temperatures.

(Conventional technology and issues to be solved) With the recent development of vacuum technology that utilizes cryogenic temperatures, space development, superconducting technology, etc., the structural materials used in these devices have a high temperature range from room temperature to cryogenic temperature. Strength is required.

Structural materials for cryogenic temperatures that have been traditionally used include:
There are austenitic stainless steels, high Mn steels, etc.
These have the disadvantage of low strength at room temperature.

As a high-strength material to replace these, the Fe-based heat-resistant alloy A2
Inconel 706 and 718, which are Ni-based heat-resistant alloys, are being considered as higher strength is required in recent years.

On the other hand, many cryogenic devices are large and have complex structures, and the structural materials used in these devices require welding.

In this regard, although the above Ni-based heat-resistant alloy has high strength,
Nb. Ti. C. Si
．． Mn. B. It is reported that reducing S etc. has the effect of increasing HAZ cracking resistance (R.G.Th
ompson; J. Matal, Vol. 4
0 (1988) p. (See 4 4). Shikashi 2,
Since Nb and T1 are precipitation strengthening elements, if these are reduced, high strength cannot be obtained.

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art and provide an Nj-based alloy for cryogenic use that has high strength from room temperature to cryogenic temperatures and has excellent weldability that does not cause H A Z cracking etc. It is something to do.

(Means for solving 11M) In order to achieve the above objective, the present inventor investigated the cause of HAZ cracking in conventional Ni-based monothermal alloys and conducted extensive research on countermeasures. As a result, we have discovered an Nj-based alloy with a new partial composition.

That is, the cryogenic Ni-based alloy according to the present invention has C:
0.0 3% or less, Si: 0.25% or less, Mn: 0
．． 25% or less, P: 0.0 1 5% or less, S: 0
．． 015% or less, Ni: 38.0-49.0%, Cr:
12.0-20.0%, Nb: 2.0-4.0%, Ti
:0.7~2.5%, Al:0.2~0.8% and MO
= more than 1.0% and 5.0% or less, and 3.5%≦
It is characterized by satisfying Nb+Ti≦((Mo/4)+4.6)%, with the remainder being Fe and impurities.

The present invention will be explained in more detail below.

(Function) The reasons for limiting the chemical components in the present invention are as follows.

C: C is necessary as an element for alloy strengthening, but 0.0
If it exceeds 3%, weldability will deteriorate, so the amount of C should be 0.0
3% or less.

Si: Si is a necessary element as a deoxidizing agent during melting, but if it is present too much, weldability is degraded, and its upper limit is 0.25%. Therefore, Sijtt is set to 0.25% or less.

Mn: Mn is a necessary element as a deoxidizing and desulfurizing agent during melting, but too much Mn is undesirable because it reduces weldability.
Its upper limit is 0.25%. Therefore, Mnffi
shall be 0.25% or less.

p, s: Both P and S reduce weldability, so 0.
0 1 5% or less.

Cr: Cr is added as a necessary element for corrosion resistance and strengthening, and is at least 12.0% in order to obtain good corrosion resistance.
The above is necessary. However, if the content is too large, the structural stability and toughness will deteriorate, and the upper limit is 20.0%. Therefore, Cri is set in the range of 12.0 to 20.0%. Nb: Nb is an unpopular element as a precipitation strengthening element.

However, if it is less than 2.0%, the effect will be small, and if it is too much, it will reduce high-temperature workability and toughness, and in particular will have an adverse effect on weldability, and the second limit is 4.0%. Therefore, N bit is set in the range of 2.0 to 4.0%.

Ti: Ti is. Like Nb, it is important as a precipitation strengthening element, but if it is less than 0.7%, its effect is small, and if it is too much, it reduces high temperature workability, toughness, and weldability, and the upper limit is 2. It is 5%.

Therefore, Tiffi is set in the range of 0.7 to 2.5%.

AQ: AQ is an element necessary for precipitation strengthening, but if it is less than 0.2%, these effects will be small, and if it is too much, it will reduce high temperature workability and toughness, and the upper limit is 0.8%. therefore. AQfJc is in the range of 0.2 to 0.8%.

Mo: MO is effective as a solid solution strengthening element, and the strength (0.2% yield strength) increases particularly at low temperatures.

Furthermore, the addition of Mo has the effect of reducing H A Z cracking during welding. Such an effect is small when the addition amount is 1.0% or less, and when it exceeds 5.0%, the toughness decreases.

Therefore, M o i exceeds 1.0% and 5. Q
% or less.

Ni: Ni is an element necessary to stabilize austenite, ensure nonmagnetism, and strengthen precipitation by γ and γ″ phases, and for this purpose, 38.0% or more is required.

However, if the Ni amount exceeds 49.0%, the structure stability and strength will decrease, so the Ni amount should be 38.0 to 4980%.
The range shall be .

The remainder is Fe and impurities. Impurities include B and the like, but it is desirable to suppress Bit to 0.0005% or less.

However, Nb. It is important that each amount of Ti satisfies the following conditions within the above range.

That is, by satisfying 3.5%≦Nb+Ti, a high room temperature yield strength of 80 kgf/mm'' or more can be obtained.

In addition, cracks in the weld heat affected zone are caused by Nb segregated at grain boundaries. Ti
is produced because it liquefies during welding, but N b + T
By satisfying i≦((Mo/4)+4.6)%, cracking of the HAZ can be prevented and excellent weldability can be obtained. Therefore, 3.5%≦Nb+Ti≦((Mo/4)+
By satisfying the condition of 4.6)%, it is possible to simultaneously satisfy excellent weldability and excellent strength at room temperature and low temperature.

Ni-based alloys with the above chemical properties are melted by conventional methods, but other than those for casting, they are manufactured by various plastic working methods such as forging, extrusion, and rolling, and are used for the construction of equipment that utilizes cryogenic temperatures. It can be applied as a material.

Next, examples of the present invention will be shown.

(Example) A 20 kg ingot was made from a test material having the chemical composition shown in Table 1 by high frequency vacuum melting, and 30 kg was made by hot forging.
It was made into a plate material with a thickness of mm.

The obtained plate material was subjected to a tensile test and a welding test.

First, the tensile test was conducted at 980°C for 3 hours as a solution treatment.
After holding, oil cooling, followed by aging treatment at 72 o6cx
After holding at 8 hr + 620°C for 8 hr, it was air cooled.

The test results are shown in FIGS. 1 and 2.

In addition, the welding test was performed on the obtained plate materials Ha 1 to Na 7,
After solution treatment, &10-Nal3 was processed into the shape shown in FIG. 3, restrained as shown in FIG. 4, and TIG welded under the conditions shown in Table 2. After welding, cross-sections of the as-welded and aged materials are cut out in a direction perpendicular to the welding direction, and the cross-sections are mirror-polished and examined with an optical microscope for the presence or absence of cracks in the weld heat-affected zone. We conducted an evaluation. The results are shown in FIG.

Based on the above test results, the following considerations can be made.

FIG. 1 shows the results of tensile tests conducted at room temperature (300 K) and liquid helium temperature (4 K).

In all test materials, the 0.2% yield strength increases as Nb+Tii increases. Generally, in precipitation-strengthened Ni-based alloys, it is known that increasing the amount of Nb and Ti increases the amount of precipitated intermetallic compounds such as γγ'' phase, thereby increasing the strength. By regulating it to 5% or more, the 0.2% proof stress at room temperature is 80
kgf/IIIn” or more, showing excellent strength.

Furthermore, Figure 2 shows the liquid helium temperature (4K), liquid nitrogen temperature (77K), and room temperature (
The results of a tensile test at 300 K) are shown. From this result. It can be seen that the addition of Mo shows excellent low-temperature strength, and that adding MO is effective when high strength is required at low temperatures.

On the other hand, from the welding test results shown in Figure 5, as the amount of Nb+Tj increases. It can be seen that there is a tendency for cracks to occur in the HAZ. but. Adding Mo has the effect of suppressing HAZ cracking.

According to FIG. 5, the relationship between the amount of MO and the amount of Ni+Ti for preventing HAZ cracking is approximately expressed by the following equation.

Nb+Ti≦((Mo/4)+4.6)%
In addition, although comparative material No. 1 0 has good weldability, as shown in FIG. 2, low-temperature strength is low.

Therefore, the amount of Mo should be regulated within the range of 1.0 to 5.0%, and 365%≦Nb+Ti≦((Mo/4)+4.6
)%, it was confirmed that the material of the present invention has excellent strength from room temperature to extremely low temperature, and does not cause cracking in the weld heat affected zone.

[Margin below 1 Nb+Ti (l, JtZ) (Effects of the invention) As detailed above, according to the present invention, high strength such as room temperature 0.2% proof stress ≧80 kgf/mn2 can be achieved from room temperature to extremely low temperature. It is possible to provide a cryogenic Ni-based alloy with excellent weldability that does not cause HAZ cracking.

[Brief explanation of drawings]

Figure 1 shows the results of a tensile test at room temperature (300K) and liquid helium temperature (4K), Figure 2 shows the results of a tensile test at room temperature (300K) and liquid helium temperature (4K), and Figure 2 shows the results at liquid helium temperature (4K) and liquid nitrogen temperature (77K).
Figure 3 is a diagram showing the shape and dimensions of the welded test piece, Figure 4 is a diagram explaining the constraint conditions of the welded test piece, FIG. 5 is a diagram showing the relationship between the amount of Mo and the amount of Nb+Ti based on the welding test results.

Claims (1)

[Claims] In weight% (the same applies hereinafter), C: 0.03% or less, Si:
0.25% or less, Mn: 0.25% or less, P: 0.01
5% or less, S: 0.015% or less, Ni: 38.0-4
9.0%, Cr: 12.0-20.0%, Nb: 2.0
~4.0%, Ti:0.7~2.5%, Al:0.2~
0.8% and Mo: more than 1.0% and 5.0% or less, and satisfies 3.5%≦Nb+Ti≦((Mo/4)+4.6)%, with the remainder being Fe and impurities. A cryogenic Ni-based alloy characterized by the following properties: