Since the gases that normally occur in steel are harmful to it ,in recent years there has been an increasing demand to reduce and control the amount of dissolved gases in steel , the steel maker aims at reducing the content of these gases below the critical threshold set of each type of steel . Hydrogen presents in steel as a residual and diffusible form, hydrogen has the highest rate of diffusion in steel.
According to the analysis report containing a high hydrogen content will have to be subjected to additional costly hydrogen annealing for an extended period of time to remove hydrogen in solid state if the reported analysis is not true a lot of time will be wasted. The aim of this study is to be measure the hydrogen content in steel by control in the measurement temperature via DH103 Leco instrument at different temperature 1100 °C , 900°C ,800°C ,700°C etc, measurement of hydrogen content in every station in SMP (EAF, LRF, VOD, CCM), compared the measurement of hydrogen between molten steel and billet and compared the measurement of hydrogen when sample taken with pin sampler and lollipop sample.
Hydrogen in steel causes various types of cracking which occur when the amount of hydrogen in a steel reaches a critical level in certain applications the hydrogen content in steel may be increased by the absorption and diffusion of atomic hydrogen produced on metal surface by a corrosion reaction for example in a wet H2S environment, this phenomenon can lead to hydrogen induced cracking (HIC), the possible source of hydrogen pick up during casting are:
Packing sand for the sliding gate -The shrouding tube for each heat a new shroud in used- Tundish covering agent
Refractories for the tundish lining- Casting powder in the surrounding air
Hydrogen can enter steel by any of several routes. The first occurs during the steel making process it self when water contained in the charge as damp scrap. Fluxes and ferro alloys in the furnace gases as inadequately dried refractories dissociates on contact with liquid steel allows hydrogen to be absorbed by the bath. This hydrogen can be largely removed by the purging action of the carbon boil but enough can remain to be trouble some later on contact between the liquid steel and moisture in ladle, refractories and humid air is another common source of hydrogen. Since this hydrogen cannot be purged by furnace reactions special techniques are required to remove it. Steel can absorb hydrogen through the action of electrochemical reactions taking place on the steel’s surface. The most common of these are pickling, electroplating, cathodic protection and corrosion, hydrogen liberates during these reactions is in part absorbed by the steel before it has the opportunity to recombine to harmless bubbles of hydrogen .absorption is favored by the presence in the electrolyte of certain poisons such as sulphides ,arsenides, phosphides and selenides which inhibit the recombination reaction .
CaO is an important addition during steel making operations, especially during ladle metallurgy .
Hydrated lime which exists as Ca(OH)2 was found to be a source of hydrogen present in the hydroxide was found to go into liquid metal.
Ca (OH)2 (S) = Ca O (S) + H2O (g)
The water vapor formed dissociates on the liquid steel surface causing hydrogen pick up by the following reaction:
H2O (g) = 2 [H] + [O]
A total 1260 g of Calcium hydroxide was added to the melt which supplies 34 moles of hydrogen, addition of metallurgical coke having close to 0.2% hydrogen content to medium carbon steel didn’t result in a hydrogen pick up by the metal. This due to loss of more than 40% of the hydrogen in the form of volatiles vapor upon heating this coke .
The diffusion coefficient of hydrogen in α iron and γiron respectively are:
D α = 2.2 x 10 –3 exp( –2900 /RT) cm2/S -D γ =1.1 X 10-2 exp( –9950/RT )cm2/S
The hydrogen content immediately before tapping of plain carbon or low alloy steel made according to normal practice is approximately as follows:
Acid open hearth 3-5 ppm – Electric arc double slag 4-8 ppm – Basic open hearth 5-9 ppm
As the steel cools the solubility of hydrogen decreased and the gas may separate in the molecular state giving rise to pressure of several 1000 atmosphere which causes cracks in the steel. These defect are commonly known as flakes. The occurrence of flakes and thus the risk of failure increases with the dimension of the steel .
Hydrogen removal the amount of hydrogen evolution is a maximum at an annealing temperature of 600 °C in actual practice the hydrogen anneal is carried out between 600 and 650 °C .The amount of hydrogen given off during annealing may be calculated according to this equation .
U = Remaining hydrogen content/ Original hydrogen content
Where D = diffusion coefficient (Cm2/S) ,
t = Time of treatment (Seconds)
r = Diameter of sample (Cm)
Hydrogen is difficult to determined by means of traditional sampling. Sample preparation and analysis the reasons being that the samples may pick up air humidity while being taken many contains pores with high hydrogen concentration and the fact that the very small hydrogen atoms continuously diffuse from the sample, the analyzed content is after but not always lower than the true hydrogen content due to diffusion.
The simplest methods for sampling applied in this work are to use an evacuated glass pipette(fig4) or cupper mould and fill it from the ingot tap or from the ladle by spoon the sample is rapidly quenched in liquid nitrogen to halt hydrogen diffusion.
At the laboratory the pin shaped sample is then carefully dried ,cut in several sample pieces while being kept at a low temperature and analyzed at best the standard deviation caused by variation in sampling , sample preparation and analysis is about 0.2 ppm which is unacceptably poor if the maximum hydrogen content is in the range of 1-1.5 ppm.
During recent years Heraus Electro Nite international N.V developed a process by which the direct and immediate measurement of hydrogen in steel, this process utilizes nitrogen carrier gas which is injected in the steel melt and then recirculated between the melt and pneumatic unit,picking up hydrogen during its passage through the steel, the process stopped when the equilibrium is reached between the hydrogen dissolved in the melt and hydrogen in the carrier gas. A thermal conductivity detector measures the final hydrogen value which is reported in PPm .
Instrument used in analysis
The instrument which has been used in analysis is Dual hydrogen (DH-103) is a signal cabinet microprocessor controlled instrument which measured the total (Diffusible and residual ) hydrogen content of steel, stainless and iron. Sample types which can be used include a standard Pyrex pin tube, Leco patened metal samplers and evacuated or static fill dual wall quartz samplers, the Pyrex or quartz samplers are placed in a reusable portable container which is attached to DH103.
The metal samplers are placed in the piercing unit .In other cases the diffused hydrogen is swept out and measured in the same manner as in residual analysis .During a residual analysis the sample is heated in a nitrogen atmosphere to release the hydrogen gas by chemical reagent. Then the hydrogen is measured by a thermal conductivity cell the final hydrogen result is shown in part per million (PPm)(6).
- The instrument temperature has been adjusted at 1100°C then calibrated instrument by using certified reference material (CRM) then measured 5 standard samples ( table 1 )
- The instrument temperature has been decreased at 900 °C , 800°C , 700 °C then measured 5 standard samples at each temperature (table 2,3,4)
- Keeping standard samples at different temperature ranges (600 °C , 500°C , 400 °C , 300°C ,200 °C )at a time 2,4 hours then calculate the percent of diffusible hydrogen from the sample ( Table 5,6,7)
- Analysis a different molten steel sample from different steel grade and compared with billet sample from the same corresponding steel grade(Table 8)
- Analysis different molten steel sample from different steel grade in different station in steel making plant (SMP)(Table 9)
- Comparison analysis of lollipop sample(fig5) (tail of CCM sample )after quenched in liquid nitrogen with pin sample of the same steel grade(Table 10)