JP7655009B2 - Mold repair welding materials and molds - Google Patents

Description

This invention relates to a mold repair welding material used for welding repair of molds and a mold that has been welded and repaired using the same.

Traditionally, hot die tool steels such as JIS SKD61 have been widely used for die casting dies and hot forging dies. When cracks or wear occur in these dies due to heat checking, the damaged parts are repaired by welding using a welding rod or welding wire in order to improve the life of the die. For this reason, die repair materials are developed with a focus on toughness, hardness, high-temperature hardness, thermal conductivity, heat checking resistance, and softening resistance, taking into account the life of the die after repair (see, for example, Patent Document 1 below).

In the case of die casting dies and hot forging dies, gas soft nitriding or salt bath nitriding is sometimes performed to form a hard nitride layer on the surface of the die in order to improve the die life. When such dies are repaired using the above repair material, blowholes (welding defects) caused by the nitride layer formed on the surface of the die occur, which causes a problem of significantly shortening the die life.
As shown in the following Patent Document 2, a welding material that takes into consideration blowhole resistance has been developed, but the hardness of the repair weld is low and it is not suitable as a material for mold repair.

JP 2017-24053 A JP 2015-110240 A

In light of the above circumstances, the present invention aims to provide a mold repair welding material that has excellent blowhole resistance and can provide appropriate hardness to the welded part, and a mold that has been welded and repaired using the same.

Claim 1 relates to a mold repair welding material, and is characterized by the composition, in mass percent, of C: 0.15-0.30%, Si: 0.20-0.50%, Mn: 0.20-0.50%, Cr: 6.0-12.0%, Mo: 0.45-1.50%, V: 0.05-0.65%, O: 0.05% or less, N: 0.10% or less, P: 0.05% or less, S: 0.05% or less, with the balance being Fe and unavoidable impurities.

In addition, the mold repair welding material may contain the following components as unavoidable impurities within the following ranges.
H≦0.0002%, Sn≦0.1%, Pb≦0.01%, Bi≦0.01%, Zn≦0.01%, Ga≦0.01%, Ge≦0.01%, Se≦0.01%, In≦0.01%, Sb≦0.01%, Te≦0.01%, Ag≦0.01%, Mg≦0.005%, Ca≦0.005%, etc.

Claim 2 is characterized in that, in claim 1, it further contains, by mass%, Al: 0.001 to 0.700%.

Claim 3 is characterized in that, in either claim 1 or 2, it further contains, by mass%, one or more of Ti, Nb, Zr, Ta, Y and Hf in a total amount of 0.40% or less.

Claim 4 is any of claims 1 to 3, further comprising, by mass%, one or two of Ni and Cu in a total amount of 1.00% or less.

Claim 5 is any of claims 1 to 4, further comprising, by mass%, one or more of the following: Co: 1.00% or less, B: 0.01% or less, and W: 1.50% or less.

Claim 6 relates to a mold, which is a nitrided mold having a surface layer with a nitrogen concentration of 0.5 mass% or more, and is characterized in that it has been welded and repaired with the mold repair welding material described in any one of claims 1 to 5.

The die repair welding material of the present invention as described above has an increased Cr content, focusing on the fact that the addition of Cr is effective in improving blowhole resistance. However, an increase in the Cr content leads to an increase in the amount of retained austenite, which reduces the hardness of the welded portion. Therefore, in the present invention, the lower limit of the C content and the amount of added carbide-forming elements are optimized in addition to increasing the Cr content, thereby ensuring the amount of C in the matrix required for appropriate hardness. The die repair welding material of the present invention not only improves blowhole resistance, but also provides the welded portion with an appropriate hardness that can suppress the occurrence of wear and cracking during use of the die, thereby extending the life of the die.
The mold repair welding material of the present invention is suitable as a repair welding material to be used for a mold made of hot work mold tool steel represented by general-purpose steel JIS SKD61, in particular as a repair welding material to be used for a mold having a nitride layer on the surface.

Next, the reasons for adding and limiting each chemical component in the present invention will be explained below.
C: 0.15-0.30%
C is an important element that determines the hardness of the welded portion, and if it is less than 0.15%, the hardness will be insufficient. Conversely, if it exceeds 0.30%, the hardness will be too hard, increasing the risk of cracking after welding. Therefore, in the present invention, C is contained within the range of 0.15 to 0.30%, with a more preferable range being 0.17 to 0.30%.

Si: 0.20-0.50%
Si is an element that plays a useful role in increasing softening resistance. When used for welding repair of hot forging dies, if the softening resistance is low, the welded portion will soften during forging. If the welded portion softens, heat checks will easily occur there. Therefore, in the present invention, the Si content is 0.20% or more in order to increase the softening resistance. On the other hand, if the Si content is more than 0.50%, the thermal conductivity decreases and heat checks will easily occur. Therefore, in the present invention, the Si content is within the range of 0.20 to 0.50%.

Mn: 0.20-0.50%
If the Mn content is less than 0.20%, the hardness will be insufficient, and if it is attempted to reduce the content to less than 0.20%, the composition of the raw materials must be considered, resulting in high manufacturing costs. On the other hand, if the Mn content exceeds 0.50%, the thermal conductivity will decrease. Therefore, in the present invention, the Mn content is set within the range of 0.20 to 0.50%.

Cr:6.0~12.0%
Cr is an essential element for improving blowhole resistance, as it is highly effective in suppressing nitrogen gasification and promoting solid solution. In order to improve blowhole resistance, the addition of 6.0% or more is necessary. However, excessive addition increases the amount of retained austenite, reducing the hardness of the welded parts. Therefore, in the present invention, Cr is contained within the range of 6.0 to 12.0%. A more preferable range is 6.8 to 12.0%.

Mo: 0.45-1.50%
Mo is an element useful for improving softening resistance. However, if it is less than 0.45%, the effect on softening resistance is small, so in the present invention, Mo is contained in an amount of 0.45% or more. Also, if it is added in an amount exceeding 1.50%, the melting cost increases. Therefore, in the present invention, Mo is contained in the range of 0.45 to 1.50%.

V: 0.05-0.65%
V plays a role in preventing grain coarsening through VC precipitation (pinning effect). If it is less than 0.05%, the amount of VC precipitation is small, making it difficult to obtain the pinning effect. Therefore, in the present invention, V is contained in an amount of 0.05% or more. Also, the more V is contained, the more V carbides are increased, improving high-temperature strength. However, if it is contained in an amount of more than 0.65%, C, which contributes to hardness, is consumed to form coarse carbides, thereby reducing the hardness and toughness of the weld. Therefore, in the present invention, V is contained within the range of 0.05 to 0.65%.

O: 0.05% or less If the O content is more than 0.05%, the amount of coarse oxides increases, which reduces the toughness of the welded portion. Therefore, in the present invention, the O content is limited to 0.05% or less.

N: 0.10% or less N promotes the formation of blowholes if the content is more than 0.10%. Therefore, in the present invention, the N content is limited to 0.10% or less.

P: 0.05% or less, S: 0.05% or less P and S are elements that promote weld cracking. Therefore, in the present invention, the P and S contents are each limited to 0.05% or less.

Al: 0.001-0.700%
Al is an element that easily forms nitrides and is useful for improving blowhole resistance, and can be added as necessary. When Al is added at 0.001% or more, nitrogen dissolved in the molten metal during welding is trapped as nitrides, suppressing the formation of blowholes. On the other hand, when it is added at more than 0.700%, it causes an increase in melting costs and a decrease in thermal conductivity. Therefore, in the present invention, the Al content is set within the range of 0.001 to 0.700%.

One or more of Ti, Nb, Zr, Ta, Y and Hf: 0.40% or less in total Ti, Nb, Zr, Ta, Y and Hf are elements that combine with carbon or nitrogen to form carbides, nitrides or carbonitrides, and contribute to suppression of grain coarsening, and can be contained as necessary. These elements generate precipitates, act as pinning particles for austenite grains, and suppress grain coarsening. In addition, they can increase toughness by forming fine grains. However, if they are contained in excess of 0.40%, C, which contributes to hardness, is consumed to form coarse carbides, resulting in reduced hardness and toughness. Therefore, in the present invention, the total content of these elements is limited to 0.40% or less.

One or both of Ni and Cu: 1.00% or less in total The addition of Ni and Cu delays the formation of pearlite and improves hardenability. However, adding more than necessary increases the cost. It also promotes the formation of blowholes. In the present invention, either Ni or Cu alone or both can be contained, but the total content of Ni and Cu is limited to 1.00% or less.

Co: 1.00% or less, W: 1.50% or less By adding Co and W, high temperature strength can be increased. However, adding more than necessary increases costs and leads to a decrease in thermal conductivity. Therefore, the Co content is limited to 1.0% or less, and the W content is limited to 1.5% or less.

B: 0.01% or less B segregates at grain boundaries to strengthen the grain boundaries and finely disperses grain boundary carbides, thereby improving toughness. However, an excessive B content can cause weld cracks, so the B content is limited to 0.01% or less.

FIG. 1 is an explanatory diagram for evaluating the blowhole resistance property.

Next, examples of the present invention will be described in detail. Test pieces were prepared using the welding rods of the examples and comparative examples (total of 39 types) shown in Tables 1 and 2 below, and various evaluations were performed.

1. Preparation of test pieces Two types of plates made of JIS SKD61 and measuring 50×150×13.5 mm were prepared.
One is a plate on which a nitride layer with a nitrogen concentration of 0.5 mass% or more is formed on the surface by performing salt bath nitriding treatment (Prevents Scoring and Scuffing (PS) treatment) by Daido DM Solution Co., Ltd. at 550°C for 10 hours. The other is a plate on which no nitride layer is formed without performing PS treatment.

For plates with a nitrided layer, TIG welding was performed on only one layer using welding rods (Φ2.0 mm x 1000 mm) with the various chemical compositions shown in Tables 1 and 2 below. For plates without a nitrided layer, five layers were overlaid by TIG welding using welding rods (Φ2.0 mm x 1000 mm) with the various chemical compositions shown in Tables 1 and 2 below. The weld length was 100 mm for each.

It is desirable for the welding material to have a diameter of 0.2 to 3.5 mm. If the diameter is thinner than 0.2 mm, more heat during welding is applied to the base material, causing a large amount of melting of the base material, which leads to the weld being harder than necessary. On the other hand, if the diameter is thicker than 3.5 mm, the heat during welding is absorbed by the welding material and is not sufficiently applied to the base material, which can cause poor fusion.

The welding conditions were as follows: Ar was used as the shielding gas, the welding rod was inserted into the arc, heated, and melted to perform welding. The welding current was 120 A and the welding speed was 5 cm/min.

2. Evaluation Using the obtained welded test pieces (plates), the blowhole resistance property and the hardness of the weld were evaluated.

<Evaluation of blowhole resistance properties>
Ten cross sections of the weld overlay of the plate on which the nitride layer was formed were cut out, and polished and corroded so that the boundary between the weld overlay and the base material could be distinguished. Then, as shown in FIG. 1, the cross sections were observed and microscopic images were taken using an optical microscope, and the total area of blowholes confirmed in the weld overlay was calculated using Win ROOF (manufactured by Mitani Shoji Co., Ltd.). The area of the blowholes was then calculated by dividing the area by the number of fields of view (10 in this case). The magnification during observation was 50 times. When the entire weld overlay could not be captured in one microscopic image, two or more images were taken and evaluated.
The criteria for judgment were as follows:
◯: Blowhole area (mm ² ) is 0.025 or less △: Blowhole area (mm ² ) is more than 0.025 to 0.030
x: Blowhole area (mm ² ) exceeds 0.030. The results are shown in Tables 1 and 2.

<Hardness evaluation>
A cross section of the weld overlay of the plate without the nitrided layer was cut out at one location, and polished and corroded so that the boundaries of each weld overlay could be distinguished. After confirming the position of the fifth weld overlay, the cross section was polished again. Then, a beaker hardness test (load 300 gf) was performed 10 times at the center of the fifth weld overlay, and the hardness (as-welded hardness) was calculated by finding the average value.
The criteria for judgment were as follows:
◯: HV420 to less than 550 △: HV400 to less than 420 ×: HV less than 400 or HV550 or more These results are also shown in Tables 1 and 2.

The evaluation results in Tables 1 and 2 reveal the following.
Comparative Example 1 is a conventionally known welding material disclosed in the above-mentioned Patent Document 1. In this Comparative Example 1, the hardness of the weld was sufficient, but the Cr amount was small and the blowhole resistance property was evaluated as "x".

Comparative Example 11 is a conventionally known welding material disclosed in Patent Document 2. This comparative example 11 is a welding material that takes into consideration blowhole resistance, and the blowhole resistance was evaluated as "○", but sufficient hardness was not obtained at the weld, and the hardness was evaluated as "×".

In Comparative Example 2, Cr was added in excess of the range specified in the present invention, and the blowhole resistance was evaluated as "○", but the hardness was evaluated as "×". It is believed that the hardness decreased due to an increase in retained austenite.

In Comparative Example 3, Ni was added in excess of the range specified in the present invention, and the hardness was rated as "○", but the blowhole resistance was rated as "×". It is believed that Ni inhibited the solid solution of nitrogen, causing an increase in blowholes.

In Comparative Example 4, the Cr content was less than the range specified in the present invention, and the blowhole resistance was evaluated as "X".

Comparative Example 5 is an example in which Cu was added in an amount exceeding the range specified in the present invention, and similarly to Comparative Example 3, the blowhole resistance was evaluated as "x".
Similarly, the blowhole resistance of Comparative Example 6, in which Ni and Cu were excessively added, was also evaluated as "x".

In Comparative Example 7, C was less than the range specified in the present invention, and the hardness was rated as "x".

Comparative Example 8 is an example in which Nb, Comparative Example 9 is an example in which V, and Comparative Example 10 is an example in which the sum of Zr and Ti were added in excess of the ranges specified in the present invention. In all of these comparative examples, the hardness was rated as "x." It is believed that the hardness was reduced due to an increase in coarse carbides and a decrease in the amount of C in the matrix.

Comparative Example 12 is an example in which C was added in excess of the range specified in the present invention, and the welded part became too hard, resulting in a hardness rating of "X."

Comparative Example 13 is an example in which N was added in excess of the range specified in the present invention, and the blowhole resistance was evaluated as "x".

In Comparative Example 14, the Mn content was less than the range specified in the present invention, and the hardness was rated as "X".

As described above, in each comparative example, at least either the hardness or the blowhole resistance was rated "X".

In contrast, in Examples 1 to 25, in which the chemical composition of the welding material (welding rod) is within the range of the present invention, both the hardness and the blowhole resistance characteristic were evaluated as "◯" or "Δ", and it is understood that these two characteristics are compatible with each other as compared with the conventionally known welding materials (Comparative Example 1 and Comparative Example 11).
The welding material of the present invention can satisfactorily suppress the occurrence of blowholes even when used to repair a nitrided mold having a surface layer with a nitrogen concentration of 0.5 mass % or more, and can also provide a repair weld with appropriate hardness.

Looking more closely at the examples, it is possible to further increase the hardness by adding more C within the range specified in the present invention, and examples in which the C content was 0.17% or more were evaluated as "○" in hardness.
Furthermore, by adding more Cr within the range specified in the present invention, the blowhole resistance can be further improved, and examples in which the Cr amount was 6.8% or more were evaluated as having blowhole resistance of "O".

In addition, when comparing Example 9 with Example 16, Example 18 with Example 14, etc., it is found that the area of the blowholes is smaller in the examples where Al was added, and that the addition of Al is effective in improving the blowhole resistance properties.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to these, and various modifications can be made without departing from the spirit of the invention.

Claims (6)

Mass % C: 0.15 to 0.30%
Si: 0.20-0.50%
Mn: 0.20-0.50%
Cr:6.0~12.0%
Mo: 0.45-1.50%
V: 0.05-0.65%
1. A mold repair welding material comprising: O: 0.05% or less; N: 0.10% or less; P: 0.05% or less; S: 0.05% or less; and the balance being Fe and unavoidable impurities. In claim 1, the composition contains, in mass%, Al: 0.001 to 0.700%
A mold repair welding material further comprising: The mold repair welding material according to claim 1 or 2, further comprising, by mass%, one or more of Ti, Nb, Zr, Ta, Y and Hf in a total amount of 0.40% or less. The mold repair welding material according to any one of claims 1 to 3, further comprising, by mass%, one or two of Ni and Cu in a total amount of 1.00% or less. The mold repair welding material according to any one of claims 1 to 4, further comprising, by mass%, one or more of the following: Co: 1.00% or less; B: 0.01% or less; and W: 1.50% or less. A nitrided mold having a surface layer with a nitrogen concentration of 0.5 mass% or more, characterized in that the mold is welded and repaired with the mold repair welding material according to any one of claims 1 to 5.