MR. RAKESH KUMAR SAHDEPT. OF INDUSTRIAL AND PRODUCTION ENGINNERINGSHAHJALAL UNIVERSITY OF SCIENCE AND TECHNOLOGY,SYLHET(BANGLADSEH)

Topic:- Hardening process of steel applied in steel industry.

Presented By:-

INTRODUCTION

Heat treatment process is a series of operations involving the heating and cooling of metals in the solid state. Hardening is one kind of heat treatment process, which is defined as rapid cooling of steel from the austenite phase. Hardening is also called as quenching, because steel is quenched from a high temperature to a liquid bath at room temperature. In the second part of this seminar we had to mansion practical knowledge about Hardening process. Due to this, we have visited two steel industries-  

Bangladesh Steel Re-Rolling Mill (BSRM) andKabir Steel Re-Rolling Mill (KSRM)

Our concern:Here we will describe:How the hardening procedure is maintained by these two industries andthereafter the resulting mechanical properties are obtained by their following process.

About BSRM & KSRM:Since BSRM and KSRM , both of them are re-rolling mill, their final product is re-rolled bar or rod which is produced by hardening & re-rolling operation from the raw material-billet.

Flow chart of Bar Production:

Billet (raw materials)

Reheating Furnace For Re-rollingReheating Temperature: 1100-1200°c

Automatic Re-rolling

Final 60 grade/75 grade Deformed Bar product

Finished Products Stacking

Delivery

A billet is a length of metal that has a round or square cross-section, with an area less than 36 sq in (230 cm2). Billets are created directly via continuous casting or extrusion or indirectly via rolling an ingot.

Billet

Quality Inspection of billet:

At the stage of billet production, during melting time dirty part of scrap floats on liquid metal that is called slag & is poured from furnace by tilting. After completing melting of scrap, sample is taken from furnace for chemical analysis and analysis is performed by spectrometer.Billet quality depends on billet chemistry. Billet chemistry is maintained into induction furnace. Sample is taken several time from induction furnace bath and analysis is performed by spectrometer to produce quality steel. The quality is ensured at all step of production by quality controller. Finally billet sample is analyzed by spectrometer for confirmation of the desired quality of billets.

Grade Information

The grade designation is equal to the minimum yield strength of the bar in ksi (1000 psi) for example grade 60 rebar has minimum yield strength of 60 ksi. Rebar is typically manufactured in grades 40, 60, and 75.

Grades specifications, that vary in yield strength, ultimate tensile strength, chemical composition, and percentage of elongation.

Case-StudyCondition to be met for quench hardening: 

Successful hardening of steel requires two conditions to be met:Firstly, the formation of homogeneous austenite when the steel is heated to correct austenitizing temperature.

And secondly, rapid cooling of austenite which will result in the transformation of marten site.

Hardening process:This process is done by two steps:

 Step 1. Heating a piece of steel &Step 2. Quenching the part

Case study 1. KSRMThey produce bar of Grade 40 & grade 60.For grade 40 rod production they follow normalizing process and For grade 60 rod production they follow the hardening process.

Step 1. Heating A furnace is a device used for heating.In KSRM , Bar Re-heating Furnace( BRF) is used: 

BRF Type Pusher type billet end-charging and side-delivery reheating furnace with one end pusher-charged and one side pusher-discharged billets. The furnace is divided into 3 sections: Preheating zone, Heating zone and Soaking (Thermal) zone.

Temperature System and Heating ModeThe BRF is triple firing zone system. The soaking zone is solid bottom structure heated by flat flame burners at top. The heating zone is overhead structure heated by both upper and lower burners

Temperature controlPre-heating zone heating zone soaking zone

1250～ 1300 1050℃ ～ 1100 . 1220℃ ～ 1280 .℃

Phase transformation during heating:   During heating, first structural change in steel begins at lower critical

temperature At this temperature pearlite changes into austenite. As the temperature is increased, more and more ferrite and cementite are

dissolved in austenite and becomes completely austenitic after reaching the upper critical temperature

Formation of homogeneous austenite occurs after holding the steel at this temperature

Step 2. QuenchingThermo Mechanically Treated (TMT)Thermo Mechanically Treated (TMT) rods are manufactured using the 'Quenching & Tempering' (Q & T) technology. Once the steel bar is rolled, it is made to pass through heat treatment over three successive stages:First stage: Intense water quenching is executed on the periphery while the core stays unchanged, leading to a martensitic surface and austenitic core. Second stage: The bar leaves the quenching box to undergo a process of 'Self Tempering' wherein heat from the austenitic core flows to the tempered martenstic surface.Final stage: 'Atmospheric Cooling' that takes place on the Cooling Bed, the austenitic core is transformed into ductile ferrite-pearlite structure. The end result is a fine combination of a strong outer rim (tempered martensite) and a ductile core (ferrite-pearlite), thereby imparting strength as well as ductility. After the quenching process, the rods are cut to desired sizes with the Shearing machines and transferred to the Cooling Bed.

TMT Box

Yield strength 60,000 psi

Ultimate tensile strength 90,000 psi

Percent elongation 10%

Quality inspection of bar products:

During the process of production hourly bar sample is taken for testing. Bar products are tested for performed mechanical properties to ensure the quality by Computerized Universal Testing Machine for yield strength, ultimate tensile strength, percent elongation.

Mechanical test result for 60 grade Deformed bar:

Case study 2. BSRM

They produce bar of Grade 40, grade 60 & grade 75For grade 40 rod production they follow normalizing process using grade 40 billet.For grade 60 rod production they also follow the normalizing process only by using grade 60 billet.And for grade 75 rod production they follow the hardening process

Step 1. Heating In BSRM , Bar Re-heating Furnace( BRF) is almost same as KSRM. The temperature control:

Zones Temperature control

Pre-heating zone 1050～ 1100℃.

heating zone 1150～ 1250℃

Step 2. QuenchingQTB (Quenching & Tempering Bar):

The Danieli, Italy QTB process at BSRM Steels Ltd.

The above schematic illustrates the exclusive DANIELI QTB process which converts low carbon steel into high strength bars with a guaranteed yield strength of 500 mpa.

How QTB process works:BSRM Xtreme rebar’s are hot-rolled from prime quality steel billets and subjected to on-line thermo-mechanical treatment in three successive stages

(a) Quenching: The hot rolled bar leaving the final mill stand is rapidly quenched by high pressure water in special venturi tubes. This hardens the surface of the bar to a depth optimized for each section through formation of martensitic rim, while the core remains hot and austenitic.

(b) Self Tempering: As the bar leaves the quenching venturi tube the core remains hot compared to the surface, allowing heat to flow from the core to the surface which causes tempering of the outer martensitic layer into a structure called Tempered Martensite. The core still remains austenitic at this stage.

(c) Atmospheric cooling: This takes place on the cooling bed, where the austenitic core is transformed into a ductile ferrite-pearlite structure. Thus the final structure consists of an optimum combination of strong outer layer (tempered martensite) with a ductile core (ferrite-pearlite).

A view of the dual phase rebar made by BSRM Steels Ltd - a hard outer shell of tempered martensite and an inner core of very tough and ductile ferrite-pearlite core. This unique technology at BSRM enables very high strength in the bar without sacrificing ductility and weldability.

In Bangladesh, BSRM is the first company to procure and implement this technology commercially and successfully.

Discussion:In this seminar there are two parts was theoretical and 2nd is practical part. In theoretical part we presented different theoretical knowledge of Hardening process. For practical part we had to mansion practical knowledges, Due to this we have visited two steel industry BSRM and KSRM. When we visited in BSRM and KSRM we got two types of quenching of hardening process using in those industries. In BSRM they are using QTB (Quenching and Tempering Bar) process and In KSRM they are using TMT (Thermo mechanical Treatment) process. After observing we found the QTB and TMT process are the similar process. In TMT box only cooling nozzle work for cooling purpose but in QTB use steeper, dryer, cooling nozzle for particular purpose. The grade of steel bar depends upon the production of billet. For a required grade chemical composition of a melting metal can be changed by adding or reducing additional alloy elements.

THANKS TO ALL