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  4. Precipitation Hardening Stainless Steels │SUS631, 632J1, TOKKIN™ 350

Precipitation Hardening Stainless Steels

SUS631, 632J1, TOKKIN™ 350

Overview

Precipitation hardening stainless steels are materials that maintain corrosion resistance*1 as stainless steel while its strength can be increased by precipitation hardening (age hardening)*2 .
In particular, SUS631 is a soft metastable austenite phase in the annealed (solution heat-treated) state. As a result, it can withstand complex machining as well as austenitic stainless steel, and its strength can be increased by precipitation hardening (heat treatment) after machining.
TOKKIN supplies three types of steel: SUS631 (17-7PH), SUS632J1 (15-7PH), and TOKKIN™ 350, and can propose steel grades according to customer applications and processing methods.

*1  The general order of corrosion resistance for each type is as follows: austenitic > precipitation hardening > ferritic.
*2  Precipitation hardening refers to the process of artificially performing aging treatment (precipitation hardening) after solution heat treatment. Typical examples of precipitation hardening are stainless steels in the 600 series and maraging steels introduced on this page.

Examples of Applications

Diaphragms, Bellows, Reed Valves, Band Saws, Mask Frame Springs, Various Springs, Bellows, etc.

Product Shapes

Product Size Range

Thickness : 0.030mm-2.0mm *Availability may vary by steel grade.

Width          : 3.0mm-300mm *Contact Sales if your desired values is over 300mm.

Length        : COIL or SHEET less than L2000mm

Features

SUS631(17-7PH) The most typical precipitation-hardening grade that can be heat-treated to increase hardness while retaining the excellent performance of 18-8 stainless steel. By applying various heat treatments, ranging from the softest solution-treated to the hardest pressure-rolled, depending on the processing and application, we can obtain materials with a strength second only to high-carbon martensitic quenched materials. In the solution heat-treated state, the elongation rate exceeds 30%, enabling strong processing such as drawing.
Please note that the material is weakly magnetic in the solution heat-treated state, but becomes quite strongly magnetic after precipitation hardening treatment.
SUS632J1(15-7PH) The solution heat-treated state (austenite phase) of SUS631 is soft and can be processed in various ways, but requires two or more heat treatments of martensitization and precipitation hardening to increase its strength. Since SUS632J1 is always in martensitic phase with or without solution treatment, it can be strengthened with only one precipitation hardening heat treatment. Solution heat-treated SUS632J1, although a martensitic phase, can be lightly machined due to its low carbon content and toughness. There are two types of finishes: solution heat-treatment finish and rolled finish. The rolled finish has higher strength, but it is not as strong as SUS631. This steel grade exhibits strong magnetism in all states.
TOKKIN™ 350
[TOKKIN Original Grade]
TOKKIN™ 350 is a Cr-Ni-Mo stainless steel. In solution annealed and annealed conditions, it boasts excellent workability and high strength due to heat treatment, and it has excellent proof stress and repeated fatigue strength. Compared to other precipitation hardening stainless steels, the addition of molybdenum provides superior corrosion resistance and oxidation resistance, and the absence of aluminium and titan provides good weldability.

Standards

Unless otherwise specified, we manufacture in accordance with JIS standards. Equivalent overseas standards are for reference only.br />
Please scroll horizontally to view.

Grade Japan USA Europe Internatinal Standards Others
JIS UNS AISI/ASTM EN ISO
SUS631
(17-7PH)
G4313, G4305 S17700 631 X7CrNiAl17-7    
SUS632J1
(15-7PH)
G4313          
TOKKIN™ 350  -  S35000  633 - -  AMS5548

Chemical Compositions

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Grade C Si Mn P S Cr Ni Mo Fe Others
SUS631
(17-7PH)
0.09 max 1.00 max 1.00 max 0.040 max 0.030 max 16.00-
18.00
6.50-7.75 - Bal. Al 0.75-1.5
SUS632J1
(15-7PH)
0.09 max 1.00-2.00 1.00 max 0.040 max 0.030 max 13.50-
15.50
6.50-7.75 - Bal. Cu 0.40-1.00
Ti 0.20-0.65
 TOKKIN™ 350 0.07-0.11 0.50 max 0.50-1.25 0.040 max 0.030 max 16.00-
17.00
4.00-5.00 2.50-3.25 Bal. N 0.07-0.13

Physical Properties

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Grade Density
[g/cm3
Specific Heat
[J/(kg・K)]
Electrical Resistance
[μΩ・cm]
Young's Modulus
[GPa]
Coefficient of Thermal Expansion
[×10-6/K]
Thermal Conductivity
[W/(m・K)]
Melting Point
[℃]
Magnetic Permeability
SUS631
(17-7PH)
7.81 420 79 200 15.3
(0-100℃)
16.3 1414-1447 Nonmagnetic in the solution heat-treated state, but becomes magnetic after processing and quite strongly magnetic after precipitation hardening treatment.
SUS632J1
(15-7PH)
7.74 502 100 196 10.9
(0-100℃)
15.9
(100℃)
- Strong
 TOKKIN™ 350 7.92 461 79 200 15.2
(25-100℃)
15.4
(100℃)
- -

Mechanical Properties

SUS631(17-7PH)

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  As annealed / As cold rolled After Precipitation Hardening Treatment
Grade Condition Temper
Designation
Hardness
[HV]
Tensile Strength
[MPa]
Elongation
[%]
V-bending
0.5t
W-bending
1.0t
Heat Treatment
Designation
Hardness
[HV]
Tensile
Strength
[MPa]
Yield
Strength
[MPa]
SUS631
(17-7PH)
Material A BA, 2B 200
max
1030
max
20
min
R90° R90° TH1050 345
min
1140
min
960
min
RH950 392
min
1230
min
1030
min
Material C 1/2H 350
min
1080
min
5
min
1.5t R90° 2.0t R90° CH 380
min
1230
min
880
min
3/4H 400
min
1180
min
- - - 450
min
1420
min
1080
min
H 450
min
1420
min
- - - 530
min
1720
min
1320
min
EH 480
min
1620
min
- - - 560
min
1900
min
1570
min
Cold Rolling and Mechanical Properties of material C and material CH
Fig. Cold Rolling and Mechanical Properties(material C・material CH)
 Precipitation Hardening Treatment Temperature and Mechanical Properties of material TH and material RH)
Fig. Precipitation Hardening Treatment Temperature and Mechanical Properties(material TH・material RH)

SUS632J1(15-7PH)

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  As annealed / As cold rolled After Precipitation Hardening Treatment
Grade Temper
Designation
Hardness
[HV]
Tensile Strength
[MPa]
Elongation
[%]
Hardness
[HV]
Tensile Strength
[MPa]
Elongation
[%]
SUS632J1
(15-7PH)
1/2H 35 max 1200 max 1250 min 400 min 1300 min 1200 min
3/4H 350 max 1450 max 1500 min 480 min 1550 min 1400 min
Cold Rolling and Mechanical Properties of SUS632J1 and SUS631
Fig. Cold Rolling and Mechanical Properties(SUS631・SUS632J1)

TOKKIN™ 350

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Grade Thickness Annealed(BA, 2B) SCT850 Condition
Hardness
[HV]
Tensile Strength
[MPa]
Yield Strength
[MPa]
Elongation
[%]
Hardness
[HV]
Tensile Strength
[MPa]
Yield Strength
[MPa]
Elongation
[%]
TOKKIN™ 350 0.030-
0.051
Negotiable* 1393 max 641 max 9 min Negotiable* 1276 min 1034 min 4 min
0.051-
0.127
Negotiable* 1372 max 627 max 10 min Negotiable* 1276 min 1034 min 6 min
0.127-
0.254
Negotiable* 1351 max 614 max 12 min Negotiable* 1276 min 1034 min 6 min
0.254-
0.508
Negotiable* 1331 max 600 max 14 min Negotiable* 1276 min 1034 min 8 min

* Hardness is provided for reference only. Please contact us for more information.

Precipitation Hardening Treatment

What is precipitation hardening treatment?

Solution treatment (solution treatment) is followed by artificial age hardening (precipitation hardening). Typical examples are the precipitation hardening stainless steels (No. 600 series) and maraging steels shown on this page.

Click here for a list of precipitation hardenable materials we supply.


The precipitation hardening heat treatment methods for each steel grade are as follows.

SUS631(17-7PH)

SUS631 is hardened from the solution treatment (annealed) condition (material A) by one of the following three methods.
procedure of precipitation hardening

The upper and middle TH1050 and RH950 treatments are hardened by a two-step heat treatment process. The first T and R treatments are primary hardening heat treatments to martensitize the metastable austenite phase A material as in quenching, while the second H treatment completely hardens the material by precipitation hardening, which is the main feature of this steel grade.
*Precipitation hardening is not possible even if only H treatment is performed in the solid solution state (Condition A material).
*TOKKIN does not have the equipment to perform TH and RH hardening treatment, so these heat treatments are performed by the customer.

The CH(900) treatment in the lower row is cold-work hardening instead of martensitizing by heat treatment, and we provide it after cold rolling to an appropriate hardness(Condition C material) .
Therefore, for customer use, only one more precipitation hardening heat treatment (H treatment) is required.

Remarks
  • The numbers at the end of TH, RH, and CH indicate the precipitation hardening treatment temperature in degrees Fahrenheit and classify the treatment.

SUS632J1(15-7PH)

Precipitation hardening heat treatment conditions for SUS632J1 are as follows:
  480°C x 1 hour holding
While SUS631 requires intermediate hardening treatment (martensification treatment), SUS632J1 can be hardened by only one heat treatment in any condition.

TOKKIN™ 350

TOKKIN™ 350 is precipitation hardened from the annealed state to obtain high strength through two treatments: primary hardening, which transforms the metastable austenite structure in the annealed state into martensite, and precipitation hardening, which completely hardens the structure.
The temperature conditions are as shown in the SCT850 heat treatment conditions below.

heat treatment of tokkin 350

Precautions for Heat Treatment

(1) Since the heat treatment temperature is quite high, it is recommended to treat in a luminous atmosphere (vacuum, H2, N2, AX gas, etc.) as much as possible.
If coloration is unavoidable, oxide scale can be removed by the following methods.
  • Low temperature scale such as H treatment: ① 10% hydrochloric acid → 30% nitric acid, or ② 15% nitric acid + 2% hydrofluoric acid (nitro fluoric acid)
  • High temperature scale such as T and R treatments: ①Caustic soda + 30% sodium nitrate molten salt → nitro fluoric acid, or ②10% caustic soda + 3% potassium permanganate boiling solution → nitro fluoric acid
  • In addition to the above, mechanical polishing, blasting, and peening are also used to remove oxide scale.

(2)Cooling of the R treatment to a lower temperature (sub zero treatment) can be easily obtained by the following method.
  • Putting a mixture of alcohol or acetone and dry ice in a heat-resistant container will produce an appropriate temperature in the vicinity of -73°C.
  • If large quantities are processed continuously, a refrigerator is used.

Comparison of Grades

Difference between SUS631 and SUS632J1

(1)Hardness increase depends on the cold rolling rate.

The following figure (left), "Cold Rolling and Mechanical Properties," shows reduction ratio-hardness and hardness after precipitation hardening.
SUS631 C (dashed line) has a large increase in hardness as the rolling ratio increases, on the opposite, SUS632J1 C (solid line), even if the rolling ratio is increased, the hardness does not increase as much as SUS631 C.
SUS632J1 has a certain degree of strength without strong cold working, and its hardness increase due to cold working is smaller, it is recommended for those who are concerned about directional properties or who require formability and punching workability.

Cold Rolling and Mechanical Properties of SUS632J1 and SUS631

(2)The amount of hardness increase due to precipitation hardening heat treatment (H treatment) differs.

When H treatment (precipitation hardening heat treatment) is applied to C material (cold-rolled material) of SUS631, the maximum increase in hardness (the higher the rolling ratio, the greater the increase in hardness after H treatment) is about HV80-90, but with SUS632J1, the increase is over HV150 regardless of rolling ratio.

Thus, for SUS631, it is possible to greatly increase the hardness by applying TH1050 or RH950 treatment to A material (soft material of HV200 or less), so we recommend A material of SUS631 if complex shapes are required.

However, for A material, martensitization treatment (T treatment, R treatment) must be performed prior to H treatment, so please pay attention to the heat treatment conditions.

Advantages of TOKKIN

Although precipitation hardening stainless steels are generally scarce in circulation, we can provide the following services;

  • Ultra-thin foil
    We can also manufacture ultra-thin foil down to 0.010 mm to 0.099 mm in thickness.
  • Small Lot
    We accept orders for minimum lot size of 100 kg or more. (Smaller lot is also available upon request.)
  • Hardness Adjustment
    Since it is made-to-order, it can be adjusted to your desired hardness. We can flexibly adjust to general 3/4H material as well as other       hardness.
  • Surface Finish
    Smooth finish or Matt Finish is available.

Materials for Bellows

We manufacture TOKKIN 350 and various other metals for bellows, and we can propose materials and specifications according to required properties such as strength, corrosion resistance, and heat resistance.

Stainless Steels for Springs

It is used for springs because it can obtain high strength through cold working and heat treatmentis. please click here to see difference of each steel grade.

We welcome your questions and inquiries regarding materials.