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Advanced QPC complex salt bath heat treatment
Abstract Salt bath surface treatment has been used widely for various industrial applications for many years. However. the technology is able to be applied in mass scale production only in limited countries. such as USA. Japan, ritain and France, due to the specific salt being obtainable only from one single company in Germany. This article reveals that a new salt. developed especially for salt bath surface treatment, has been produced successfully in China offering superior surface properties and distinct benefits such as: the elimination of the need to pass air into the nitriding salt bath and the use of a centrifugal pump in the oxidation stage; simple equipment; low cost; and ease of operation. This paper also describes rhe effects of temperature and process duration on the hardness, fatigue. corrosion resistance and wear resistance of the treated components. ,C 1997 Elsevier Science S.A. Keyrcords: QPQ: Salt bath heat treatment; Surface treatment _______
The QPQ complex [I-S] salt bath treatment is a type of very advanced technology for surface strengthening. It is used widely in the field of manufacturing engineering and in the mechanical and automobile industry to increase surface wear resistance, to enhance fatigue strength and to improve the corrosion resistance of the treated components. The QPQ process is particularly suitable for all kinds of low-carbon steel, cast iron and powder-metallurgy components. After QPQ treatment, plain-carbon steel can be used instead of stainless steel, whilst the QPQ process can also substitute for anti-corrosion and anti-oxidizing processes such as blacking, phosphorescing, chromium plating, etc.
2. Experimental work 2. I. The principle of the QPQ sut~L1Ce-fl.eCtttttettt p?-OWSS
The QPQ surface-treatment process consists of six steps: (1) degreasing, (2) preheating, (3) nitriding, (4) oxidizing, (5) washing, and (6) oil immersion. The success of the QPQ process depends largely on the control of the nitriding process. The concentration of 09240136/97/$17.00 0 1997 Ekevier Science S.A. All rights reserved. HI SO924-0136(96)02535-6
-__-___
~_
__
the CNO _~ released or decomposed from the nitriding salt must be sufficiently high, whilst the concentration of CN must be keep as low as possible. The nascent nitrogen used for the nitriding reaction comes from the dissociation of CNO - : 4CN0
+ CO;
+ 2CN0
+ CO t- 2[N]
0)
In order to keep the CN - concentration to a minimum, a specially formulated oxidizing agent is added to the salt to react with the CN _ to form 2CNO- : 2CN-
+OZ+2CNO-
(2)
The selection and the concentration of the oxidizing agent are very critical: if the oxidization reaction is too weak, the content of CN - cannot be reduced signihcantly, whilst if the oxidizing reaction is too vigorous, the decomposition of CNO - will become too rapid which in turn will affect the stability of the salt bath process. By means of very many lengthy experiments, a nitriding salt with a special formula containing the best propor-tc
C.F. Yeung et al. / Jowwl of Materials Pfocewbg Terholog_v 66 (1997) 249-252
250
Fig. 1. Relationship between the concentration (%) and the treatment time.
of CNO - (W, CN Fig. 2. Effect of oxidizing salt on CN - deposition.
permissible National Environmental dard.
2.2. Oxidizirfg agent
After nitriding treatment, the workpiece has traces of CN- on the surface. The function of the oxidizing agent is to remove such residual CN -, in so doing leaving a black oxide layer on the workpiece surface which increases the corrosion resistance and improves the cosmetic appearance of the workpiece. From experiment, it is found that the decomposition of CN - is affected by temperature and time (see Fig. 2). From Fig. 2, processing at 300°C for 15 min, the concentration of CN- decreases from 100 to 0.5 mg kg-‘. In actual operation, the oxidizing process takes place at 350-370°C and the process duration ranges from 15 to 20 min. As a result of the oxidization treatment, the content of the CN- on the workpiece is so low that the water used for resin needs no special treatment and can be drained directly. The Chinese Environmental Protection Department had tested and verified that the waste water contains CN- to a level of only one tenth of the
Protection Stan-
2.3. Hardness testing ln order to compare the influence of the QPQ process on the material, 15 different types of material were selected and treated with the QPQ surface treatment process at 57O”C, nitriding for 2 h with 37% CNO and oxidizing for 15 min at 350°C. The surface hardness and the depth of the composite layer are listed in Table 1.
2.4. Wear-resistawe
determination
Wear resistance tests was carried out on a Type MM 200 wear resistance tester for 2 h with a sliding rate of iOO%.a speed of 200 strokes min - ’ and a load of 200 kgf (1.762 kN). The material of the specimens used for the testing was AISI 4140 chromium steel (0.4% C and
Table I The surface hardness and the depth of the composite layer Sample number
Material
Surface hardness (HV 0.1 g)
Thickness of nitrided layer (pm)
I 2 3 4 5 6 I 8 9 IO 11 12 13 14 15
Pure iron A3 AlSI 1020 AISJ 4120 AISI 1045 AISI 4140 WI AMS 6438 AMS A681 AMS 6470 Q2 321 M2 annealed M2 (hardened and tempered) Ductile cast iron
650 661 681 172 642 772 661 772 946 946 824 1003 824 1533 606
20 18 18 12 15 I5 12 9 6 0 9 9 6 6
Table 2 Experimental results of the w,%ar-resistance test
QpQ Hard-chromium
650 8 13
0.X 0 46
1 21
plating Ion-nitriding Induction hard-
700 720
0.62 5.18
3.8 23.7
ening Conventional
660
6.46
29.4
hardening __i0'
IO”
N (Namkerolcycks
Logscale
to !Uire)
1.O’%Cr). Five specimens, of size 10 mm x 10 mm. were tested, the results being listed in Table 2.
Four different materials, with three samples each, st?.ndard by salt spray according to AS were d at a 7), the tests being cond (AST temperature of 35 + 2°C under humidity greater than 95% with 5% NaCl water solution sprayed continuously until t-us? appeared, the results nf the ?ests being shown in Table 3. 2.6. Fatigue testipzg
Two sets of specimen with 5 samples each. made from 1045 and treated with QPQ and hardening and high temperature tempering, vl’ere tested using a bend fatigue machine (model: PQl bend). Standard specimens with a diameter of 7.5 mm were rotated at 300 r.p.m., their fatigue strength vs., number of cycles to failure being shown in Fig. 3. The QPQ-treated sample has a fatigue strength of 570 IMPa, which is 40% greater than the value for hardening plus high temperature tempering of the same material, of 405 MPa.
Fig. 4 shows the micro-structure of 1045 steel after PQ treatment, where the treated s.urface is seen to consist of three layers: (i) the oxide film layer, which increases the corrosion resistance, improves the cosmetic appearance and contains oil which reduces friction; (ii) the nitrided layer or the white layer, which improves the wear-resistance and the corrosion resistance; and (iii) the diffused layer. which increases the fatigue strength.
Fig. 5 shows the influence of the nitriding temperature on the hardness of the nitrided layer at a constant nitriding time (2 h), from which figure the following can be seen. 1. At the temperature 550°C, the peak hardness occurs at the outer-most layer. ?_. At the temperature range from 560-590°C an increase of temperature will increase t.he peak hardness value.
Table 3 Result of the salt-spray
tests Rust starting
Relative corrosively
:ime (h)
(hl
1045 QPQ treated
140
Stainless steel (321) 1045 decorative
28 4
_
I i,‘5 I,‘35
chromium plated D2 (chromium steel
3.5
I ,‘40
1013) 1045 with blacking
0.5
11140 Fig. 4. Microstructure of 1045 steel after the QPQ treatment
C.F. Yeung et al. /Journal of Mater&
252
600
-
500
-
400
-
300
-
Processing Technology 66 (1997) 249-252
200
zoo -
-
Carcbrrdaers
Corn brdnsu 0.01 0.01
0.02
0.03
0.04
0.05
o,w
Fig. 5. The effect of the nitriding temperature
Swfaw
diaaoce(mm)
4. Conclusions Through the evidence provided by the above reported experimental results, the QPQ process can be
0.03
004
-0.05
I 0.06
smrrace diswace (mm)
Fig. 6. The effect of nitriding time on the hardness.
on the hardness.
3. When the temperature exceeds 56O”C, the peak hardness shifts from the outer-most layer to the inner nitrided layer because the structure of outer layer becomes less dense. Fig. 6 shows the influence of the nitriding time on the hardness of the nitrided layer at a constant temperature of 57O”C,from which figure the following is noted. 1. Increase in the nitriding time increases the peak hardness value, the hardness value initially increasing proportional to the duration. until it reaches a maximum of 750 HV after 3 h, thereafter remaining unchanged. 2. Apart from the time issue, the peak hardness shifts from the outer-most layer to the inner nitrided layer because the structure of the outer layer becomes less dense. If the structure of the nitrided layer becomes less dense, the mechanical properties will become poor there. Therefore, the optimum QPQ processing temperature is chosen as 570°C.
0.02
regarded as a highly satisfactory process by means of which to perform surface treatment. The corrosion resistance offered by the QPQ process is equivalent to that of Type 321 stainless steel. The QPQ process not only provides a cost-effective alternative for surface treatment, but has the very significant benefit that the process does not produce poisonous by-products. The process can thus be adopted widely in most manufacturing industries.
References 111A.
Jones and P.J. Evans, Heat Treatment qf‘Mets., 20 (4) (1993) 99- 100. 121B.X. Liu, T.L. Xing and J.H. LIU, Heat Treutnlent qf Mets., (Jinshu Rechuli) (I ) (1994) 20- 23. PI G.A. Collins, R. Hutchings, K.T. Short and J. Tendys, Heat Treatment of Mets., 22 (4) (1995) 91-94. [41 G. Scavino, M. Rosso, G. Ubertalli and A. Balzarottt, ht. J. Mats Protenol.. 8 (2-4) (1993) 290-302. VI H.E. Chandler, Metal Prog., 128 (4) (1985) 43-46. WI H.E. Chandler, Metal Prog., 130 (2) (1986) 19-23. 171N. Dingremont, E, Berggmann, M. Hans and P. Collignon, Proc. 22nd ht. Conf. on Metallurgical Coatirzgs and Thin Films, Surface and Coatings Tech., 1995, Vol. 76-77 (l-3) part 1, Nov. 1995, pp. 218-224. [S] P. Stratton, Mats. World, 3 (11) (1995) 530-533.
The QPQ complex [I-S] salt bath treatment is a type of very advanced technology for surface strengthening. It is used widely in the field of manufacturing engineering and in the mechanical and automobile industry to increase surface wear resistance, to enhance fatigue strength and to improve the corrosion resistance of the treated components. The QPQ process is particularly suitable for all kinds of low-carbon steel, cast iron and powder-metallurgy components. After QPQ treatment, plain-carbon steel can be used instead of stainless steel, whilst the QPQ process can also substitute for anti-corrosion and anti-oxidizing processes such as blacking, phosphorescing, chromium plating, etc.
2. Experimental work 2. I. The principle of the QPQ sut~L1Ce-fl.eCtttttettt p?-OWSS
The QPQ surface-treatment process consists of six steps: (1) degreasing, (2) preheating, (3) nitriding, (4) oxidizing, (5) washing, and (6) oil immersion. The success of the QPQ process depends largely on the control of the nitriding process. The concentration of 09240136/97/$17.00 0 1997 Ekevier Science S.A. All rights reserved. HI SO924-0136(96)02535-6
-__-___
~_
__
the CNO _~ released or decomposed from the nitriding salt must be sufficiently high, whilst the concentration of CN must be keep as low as possible. The nascent nitrogen used for the nitriding reaction comes from the dissociation of CNO - : 4CN0
+ CO;
+ 2CN0
+ CO t- 2[N]
0)
In order to keep the CN - concentration to a minimum, a specially formulated oxidizing agent is added to the salt to react with the CN _ to form 2CNO- : 2CN-
+OZ+2CNO-
(2)
The selection and the concentration of the oxidizing agent are very critical: if the oxidization reaction is too weak, the content of CN - cannot be reduced signihcantly, whilst if the oxidizing reaction is too vigorous, the decomposition of CNO - will become too rapid which in turn will affect the stability of the salt bath process. By means of very many lengthy experiments, a nitriding salt with a special formula containing the best propor-tc
C.F. Yeung et al. / Jowwl of Materials Pfocewbg Terholog_v 66 (1997) 249-252
250
Fig. 1. Relationship between the concentration (%) and the treatment time.
of CNO - (W, CN Fig. 2. Effect of oxidizing salt on CN - deposition.
permissible National Environmental dard.
2.2. Oxidizirfg agent
After nitriding treatment, the workpiece has traces of CN- on the surface. The function of the oxidizing agent is to remove such residual CN -, in so doing leaving a black oxide layer on the workpiece surface which increases the corrosion resistance and improves the cosmetic appearance of the workpiece. From experiment, it is found that the decomposition of CN - is affected by temperature and time (see Fig. 2). From Fig. 2, processing at 300°C for 15 min, the concentration of CN- decreases from 100 to 0.5 mg kg-‘. In actual operation, the oxidizing process takes place at 350-370°C and the process duration ranges from 15 to 20 min. As a result of the oxidization treatment, the content of the CN- on the workpiece is so low that the water used for resin needs no special treatment and can be drained directly. The Chinese Environmental Protection Department had tested and verified that the waste water contains CN- to a level of only one tenth of the
Protection Stan-
2.3. Hardness testing ln order to compare the influence of the QPQ process on the material, 15 different types of material were selected and treated with the QPQ surface treatment process at 57O”C, nitriding for 2 h with 37% CNO and oxidizing for 15 min at 350°C. The surface hardness and the depth of the composite layer are listed in Table 1.
2.4. Wear-resistawe
determination
Wear resistance tests was carried out on a Type MM 200 wear resistance tester for 2 h with a sliding rate of iOO%.a speed of 200 strokes min - ’ and a load of 200 kgf (1.762 kN). The material of the specimens used for the testing was AISI 4140 chromium steel (0.4% C and
Table I The surface hardness and the depth of the composite layer Sample number
Material
Surface hardness (HV 0.1 g)
Thickness of nitrided layer (pm)
I 2 3 4 5 6 I 8 9 IO 11 12 13 14 15
Pure iron A3 AlSI 1020 AISJ 4120 AISI 1045 AISI 4140 WI AMS 6438 AMS A681 AMS 6470 Q2 321 M2 annealed M2 (hardened and tempered) Ductile cast iron
650 661 681 172 642 772 661 772 946 946 824 1003 824 1533 606
20 18 18 12 15 I5 12 9 6 0 9 9 6 6
Table 2 Experimental results of the w,%ar-resistance test
QpQ Hard-chromium
650 8 13
0.X 0 46
1 21
plating Ion-nitriding Induction hard-
700 720
0.62 5.18
3.8 23.7
ening Conventional
660
6.46
29.4
hardening __i0'
IO”
N (Namkerolcycks
Logscale
to !Uire)
1.O’%Cr). Five specimens, of size 10 mm x 10 mm. were tested, the results being listed in Table 2.
Four different materials, with three samples each, st?.ndard by salt spray according to AS were d at a 7), the tests being cond (AST temperature of 35 + 2°C under humidity greater than 95% with 5% NaCl water solution sprayed continuously until t-us? appeared, the results nf the ?ests being shown in Table 3. 2.6. Fatigue testipzg
Two sets of specimen with 5 samples each. made from 1045 and treated with QPQ and hardening and high temperature tempering, vl’ere tested using a bend fatigue machine (model: PQl bend). Standard specimens with a diameter of 7.5 mm were rotated at 300 r.p.m., their fatigue strength vs., number of cycles to failure being shown in Fig. 3. The QPQ-treated sample has a fatigue strength of 570 IMPa, which is 40% greater than the value for hardening plus high temperature tempering of the same material, of 405 MPa.
Fig. 4 shows the micro-structure of 1045 steel after PQ treatment, where the treated s.urface is seen to consist of three layers: (i) the oxide film layer, which increases the corrosion resistance, improves the cosmetic appearance and contains oil which reduces friction; (ii) the nitrided layer or the white layer, which improves the wear-resistance and the corrosion resistance; and (iii) the diffused layer. which increases the fatigue strength.
Fig. 5 shows the influence of the nitriding temperature on the hardness of the nitrided layer at a constant nitriding time (2 h), from which figure the following can be seen. 1. At the temperature 550°C, the peak hardness occurs at the outer-most layer. ?_. At the temperature range from 560-590°C an increase of temperature will increase t.he peak hardness value.
Table 3 Result of the salt-spray
tests Rust starting
Relative corrosively
:ime (h)
(hl
1045 QPQ treated
140
Stainless steel (321) 1045 decorative
28 4
_
I i,‘5 I,‘35
chromium plated D2 (chromium steel
3.5
I ,‘40
1013) 1045 with blacking
0.5
11140 Fig. 4. Microstructure of 1045 steel after the QPQ treatment
C.F. Yeung et al. /Journal of Mater&
252
600
-
500
-
400
-
300
-
Processing Technology 66 (1997) 249-252
200
zoo -
-
Carcbrrdaers
Corn brdnsu 0.01 0.01
0.02
0.03
0.04
0.05
o,w
Fig. 5. The effect of the nitriding temperature
Swfaw
diaaoce(mm)
4. Conclusions Through the evidence provided by the above reported experimental results, the QPQ process can be
0.03
004
-0.05
I 0.06
smrrace diswace (mm)
Fig. 6. The effect of nitriding time on the hardness.
on the hardness.
3. When the temperature exceeds 56O”C, the peak hardness shifts from the outer-most layer to the inner nitrided layer because the structure of outer layer becomes less dense. Fig. 6 shows the influence of the nitriding time on the hardness of the nitrided layer at a constant temperature of 57O”C,from which figure the following is noted. 1. Increase in the nitriding time increases the peak hardness value, the hardness value initially increasing proportional to the duration. until it reaches a maximum of 750 HV after 3 h, thereafter remaining unchanged. 2. Apart from the time issue, the peak hardness shifts from the outer-most layer to the inner nitrided layer because the structure of the outer layer becomes less dense. If the structure of the nitrided layer becomes less dense, the mechanical properties will become poor there. Therefore, the optimum QPQ processing temperature is chosen as 570°C.
0.02
regarded as a highly satisfactory process by means of which to perform surface treatment. The corrosion resistance offered by the QPQ process is equivalent to that of Type 321 stainless steel. The QPQ process not only provides a cost-effective alternative for surface treatment, but has the very significant benefit that the process does not produce poisonous by-products. The process can thus be adopted widely in most manufacturing industries.
References 111A.
Jones and P.J. Evans, Heat Treatment qf‘Mets., 20 (4) (1993) 99- 100. 121B.X. Liu, T.L. Xing and J.H. LIU, Heat Treutnlent qf Mets., (Jinshu Rechuli) (I ) (1994) 20- 23. PI G.A. Collins, R. Hutchings, K.T. Short and J. Tendys, Heat Treatment of Mets., 22 (4) (1995) 91-94. [41 G. Scavino, M. Rosso, G. Ubertalli and A. Balzarottt, ht. J. Mats Protenol.. 8 (2-4) (1993) 290-302. VI H.E. Chandler, Metal Prog., 128 (4) (1985) 43-46. WI H.E. Chandler, Metal Prog., 130 (2) (1986) 19-23. 171N. Dingremont, E, Berggmann, M. Hans and P. Collignon, Proc. 22nd ht. Conf. on Metallurgical Coatirzgs and Thin Films, Surface and Coatings Tech., 1995, Vol. 76-77 (l-3) part 1, Nov. 1995, pp. 218-224. [S] P. Stratton, Mats. World, 3 (11) (1995) 530-533.