Introduction

The information presented here refers to a test run on two continuous and three batch STC furnaces performing a wire and rod spheroidising annealing process. The first phase of the test employing the two continuous roller hearth furnaces compared results in terms of achieved mechanical and metallurgical properties. The second phase of the test performed on these furnaces consisted in decreasing the time cycle and comparing the mechanical and metallurgical properties with those established in phase one. This improvement in the time cycle is especially beneficial in fastener production.

The third phase of the test examined wire and rod spheroidising annealing in STC roller hearth batch furnaces, and the results were compared with those obtained previously. The steel grades investigated were USA standard 4037, 1541, and 10B21 (hot-rolled and drawn). This article presents results from steel grade 4037 in the hot-rolled condition.

Roller hearth furnaces

Roller hearth furnaces employed for wire and rod spheroidising annealing are of the batch and continuous-operation type. The layout, material grades, and production mix and requirements are the parameters that govern the choice of one over the other. Typically, continuous operation furnaces are required when production of the same material grade (or within the same “family” of steel grades heat treatable with the same thermal cycle) exceeds 1,500 tonnes per month. When the mix is broader, and production requirements are below 1,500 tonnes a month, the batch furnace in single or multiple installation is preferred. All the furnaces studied for this article use endothermic gas, supplied by a generator, as the protective atmosphere. They are heated by radiant tube burners firing natural gas. The continuous furnaces are rated at 3,000 tonnes per month and have a multi-zone temp erature and carbon potential control; the batch furnaces also have temp erature and carbon potential control, and 28-tonne capacity each.

The main operational differences between the two furnace types are:

• In the continuous furnace, a single-coil tray system is loaded in the furnace with a frequency based on time cycle. The coil-tray system travels at a fixed speed along the furnace chamber performing the thermal cycle;
• In the batch furnace, multiple coils are placed on an extended tray and loaded at once into the furnace. The load performs the thermal cycle in a static mode.

The furnaces under examination have been in operation for almost three years, and several tests for product quality and furnace efficiency have been run during this period. The cold heading industry is always focused on zero free ferrite and recarburisation, minimum allowed depth of partial decarburisation, and minimum product variability in terms of such mechanical properties as hardness, tensile strength, and reduction of area. The use of endothermic gas produced by an endothermic gas generator (40% H 2, 40% N 2, 20% CO) allows processing of the wire and rod coils without any carburisation or decarburisation. This is achieved by means of strict harmonized control of each zone temp erature and carbon potential set-point, according to the steel grade recipe.

Test procedure

The alloy used in testing, AISI 4037, was chosen because it is used extensively in the fastener industry (Table I). The alloy was hot rolled, cleaned in HCl, and light lime-coated. Samples were taken from the top, middle, and bottom of each coil in the as-rolled and as-spheroidised condition (Table II). In each condition the mechanical properties, tensile strength, hardness, and reduction of area were investigated, as well as the microstructure in terms of free ferrite depth, partial decarb depth, and spheroidisation rate.

Material data:

Continuous roller hearth furnace

Table I: AISI 4037 rod: chemical composition

Material data:

Continuous roller hearth furnace

A temp erature recording system checked the coil temp erature uniformity. The recording system was placed inside the coil for the continuous furnace and along the tray load for batch furnace. The system consists of:

• One thermal barrier;
• One heat sink;
• Eight channel thermocouples data logger;
• Programmable software.

The software was programmed to accord with the furnace design. The standards applied in investigating product properties are:

• Hardness: ASTM E18-98, 150kg load
• Tensile Strength: ASTM E8-99
• Spheroidisation rate and decarburisation: ASTM A892-98
• Decarburisation: IFIS 140 – Transverse section, picral etch 200x
• Spheroidising rating method: for all spheroidised rod/wire, view at least three fields and rate the percentage of spheroids and lamellar pearlite. All spheroids and one-half of pinpoints give the spheroidisation rating (transverse section picral etch 1,000x).

Material Data:

Batch Roller Hearth Furnace  

Table III: AISI 4037 rod: chemical composition

Material Data:

Batch Roller Hearth Furnace

Table IV : AISI 4037 rod: coil data

Results: continuous roller hearth furnace

The tests have been identified as follows:

• Group A: test at 18 hours cycle;
• Group B: test at 17 hours cycle;
• Group C: test at 17 hours cycle with modified recipe.

In this article we present for group A, tests 6 and 7; for group B, test 5; and for group C, test 9. The first step consists in running tests in the continuous furnaces to confirm the results achieved in the previous operation. Test 6 and test 7 were run in furnace N°1 and furnace N°2, using the AISI 4037 standard recipe of 18 hours cycle duration. Test 7 was run for AISI 4037 with a higher chromium percentage (Table I). This increased chromium percentage is responsible for higher values of hardness and tensile strength. Wire/rod results for AISI 4037 in six months of operation are as follows:

• Steel grade: AISI 4037;
• Samples: 332;
• Hardness RB: min 71.5 - max 83.7;
• Tensile (ksi): min 63.3 - max 79.8;
• Reduction of area %: min 50 - max 71.5.

In the second step of the test the time cycle was decreased from 18 hours to 17 hours. The third step consisted of maintaining a recipe time cycle of 17 hours and comparing it with the standard recipe run for AISI 4037 in a batch roller hearth furnace. This was done to establish the extent to which a result comparison could be significant.

Group B - Test 5

The product (hot rolled) was annealed for 17 hours in furnace N°2. The prior annealing structure consists of ferrite and fine and coarse pearlite. The spheroidised annealed condition structure consists of spheroids in a ferrite matrix. The spheroidising rate was evaluated at 90%. No decarburisation is added by the spheroidizing process.

Group A - Test 6

The product (hot rolled) was annealed for 18 hours in furnace N°1. The prior annealing structure consists of ferrite and fine and coarse pearlite. The spheroidised annealed condition structure consists of spheroids in a ferrite matrix. The spheroidising rate was evaluated at 90%. No decarburisation is added by the spheroidising process.

Group A - Test 7

The product (hot rolled) was annealed for 18 hours in furnace N°2. The prior annealing structure consists of ferrite and fine pearlite. The spheroidised annealed condition structure consists of spheroids in ferrite matrix. The spheroidising rate was evaluated at 90%. No decarburisation is added by the spheroidising process.

Group C - Test 9

The product (hot rolled) was annealed for 17 hours in furnace N°1. The prior annealing structure consists of ferrite and fine pearlite as shown Figure 2 (200 x picral etch) and Figure 1 (1,000 x picral etch). The spheroidised annealed condition structure consists of spheroids in ferrite matrix. Figure 4 (200 x picral etch) and Figure 3 (1,000 x picral etch). The spheroidising rate was evaluated at 90%. No decarburisation is added by the spheroidising process.

Figure 1: Prior annealing at 1,000 x picral etch.

Figure 2: Prior annealing at 200 x picral etch.

Figure 3: Spheroidised condition at 1,000 x picral etch.

Figure 4: Spheroidised condition at 200 x picral etch.

Thermocouples temp erature profile: continuous roller hearth furnace

Most rod and wire of various steel grades is spheroidised in the inter-critical temp erature range, with the exception of low-carbon rod and plain wire, which are spheroidised subcritically. The rates of spheroidisation achievable depend on prior microstructure, and is greatest for quenched structures in which the carbide phase is fine and dispersed and for prior cold worked material. Regardless of which thermal cycle is used to produce a spheroidised microstructure, the microstructure of prior-annealed steel has an important bearing on whether spheroids will form. The microstructure of hot rolled wire is a function of the rolling practice. The ferrite grain size and pearlite colony are determined by the hot finishing, while inter-lamellar pearlite spacing is determined by the cooling rate following the last roll stand.

The wire rolling industry has moved in the direction of faster cooling, since finer pearlite leads to a shorter spheroidising cycle. The typical thermocouples profile downloaded from the recording system for test 6 is shown in Figure 5. The thermocouples profile shows the typical thermal cycle for spheroidised-annealing of AISI 4037 in an 18 hours cycle. Starting from ambient temp erature, the product is heated up to the soaking temp erature in the intercritical temp erature range: here the spheroidal nuclei are formed during intercritical soak, and these subsequently grow into larger spheroids as austenite decomposes in the slow cooling phase. .

Figure 5: Coil thermocouples profile for test 6

At the end of the slow cooling ramp, after having reached transformation temp erature (approximately 650°C), the product is discharged from the furnace. It is important to note that, for operating in the intercritical range, close control of temp erature is necessary because time and temp erature affect austenitisation. They thereby influence the number of undissolved carbides from which nucleation and coalescence of the spheroid occur. For this reason, an extensive soaking period is usually employed.

The values in the as-spheroidised condition are consistent with the IFI 140 standard and present minimum variability. In detail, the higher Cr content for AISI 4037 in test 7 is responsible for the higher value of tensility and hardness in the spheroidised condition. From the comparison of test 6 and test 5 (18-hour cycle versus 17-hour cycle), the product maintains the same quality characteristics as with the shorter time cycle. To confirm the results obtained during laboratory testing, further investigations have checked the coil temp erature uniformity at the end of the soaking period (Figures 6, 7, 8).

Figure 6: Hardness before and after spheroidising

Figure 7: Tensile strength before and after spheroidising

Figure 8: Reduction of area prior and after spheroidising

Close control of temp erature is required in order to dissolve the carbides from which nucleation and coalescence of spheroids occur. Analysis of the coil temp erature at the end of the soaking period shows how the tight furnace temp erature control and furnace temp erature uniformity reflect on coil temp erature distribution. We have compared the difference in temp erature between two coils in two furnaces running according to the same recipe (Figure 9) and two coils in two furnaces running with different time cycle (Figure 10).

Figure 9: Test 6 versus Test 7 at 18 hours cycle

At the end of the soaking period the maximum temp erature difference between four probes of test 6 and the equivalent probes in test 7 is 6.3°F (3.5°C) for the two coils in two different furnaces under the same recipe.

Figure 10: Test 5 versus Test 10. 6 - 17 hours cycle versus 18 hours cycle

 By changing the recipe time cycle, the maximum difference in temp erature between five probes of test 5 and the equivalent probes in test 6 is 8.7°F (4.7°C) for two coils in two different furnaces at a different time cycle. This result, confirmed by additional tests run on the same furnaces, demonstrates that it is possible to shorten the cycle in a defined time range, while maintaining product quality and increasing the production rate.

Batch roller hearth furnaces (STC)

Three 28 tonne capacity roller batch furnaces were used for comparison-testing with continuous furnaces. We present here the results achieved in terms of mechanical properties.

Continuous furnaces versus batch furnaces

Our comparisons show that mechanical properties for the same steel grade in a spheroidising cycle do not relate to the type of furnace. The results achieved with the temp erature recording system were compared for test 9 in continuous roller hearth furnace N°1 and test 3 in batch furnace N°4. The maximum temp erature difference between coil thermocouples at the end of the soaking zone is 8°F (4.4°C) for the two furnaces and within one coil of the continuous furnace and the entire load of the batch furnace.

Tensile strength in the spheroidised condition was compared by reference to the continuous furnace tests 5, 6, and 9 and batch furnace tests 1, 2, and 3. The tensile strength (Figure 11) in the spheroidised condition shows a minimum value of 69.2ksi and a maximum of 73.8ksi on 63 samples. The mean value is 71.1 with a standard deviation of 1.1 and a normal distribution (Figure 12).

Figure 11: Tensility: comparison batch versus continuous furnace

The hardness (RB) in the spheroidised condition shows a minimum value of 70.1ksi and a maximum of 78.5ksi on 63 samples. The mean value is 74.7 with a standard deviation of 1.8. The reduction of area in the spheroidised condition shows a minimum value of 63.5 and a maximum value of 69.4 on 68 samples. The mean value is 67.3 with a standard deviation of 0.98.

Figure 12: Tensile strength distribution

Summary

In conclusion we wish to highlight what these results might mean for the cold finishing industry and the cold heading industry.

Cold finishing industry: The high- temp erature uniformity at the end of the soaking zone (usually <3°C) will positively affect the product in terms of low mechanical property variability and a high spheroidisation rate. The metallurgical results are consistent with standard requirements in terms of zero free ferrite, and below what is required for partial decarburisation depth. This results in high product quality and high production efficiency with no product rejection. It is possible to shorten the cycle in a certain time range and still maintain product quality. This represents a great benefit to the cold finishing processor because the product can be heat treated at lower energy costs for the entire cycle and at a higher production rate. This promises the heat treater a higher profit margin; or, the product can be offered at a lower price. Continuous roller hearth and batch type furnaces operating in an endothermic gas atmosphere present no differences in terms of quality.

Cold heading industry: The wire and rod quality is the most important issue. Since our results are in accordance with the standard requirements (maximum hardness, tensile strength, decarburisation, and spheroidisation), variable parameters govern wire and rod formability in the production of fasteners. Low variability of spheroidised wire and rod allow for high quality and homogeneous fastener products, easier set-up of machining equipment for cold heading, and longer tool life.

References

• James O’Brien and William F. Hosford: “Spheroidising of Medium Carbon Steel”, Industrial Heating, September 2000, pp 79-84.
• ASM International, Material Engineering Institute, Heat Treatment of Steel - Course 0733, Lesson 4: “Annealing and Normalising of Steel”, pp 9-12.
• Fabrizio Pere, “Spheroidise Annealing in Continuous Roller Hearth Furnaces”, Vol. 2, September 2000.
• Fabrizio Pere, “Spheroidise Annealing in Batch Roller Hearth Furnaces”, Vol. 1, November 2000.
• Fabrizio Pere and Bernard A. Jakicic, “Forno a rulli tipo “STC” per trattamenti termici”, 17 th Convegno Nazionale Trattamenti Termici, Salsomaggiore, May 5-7, 1999, pp 239-249.
• Robert L. Draker and Krish Naidu, “Control of Surface Carbon During Intercritical And Subcritical Annealing”, Wire Journal International, January 2000, pp 96-103.